I just saw this and it's late, so my response will be even later. But yeah. I can give some tips on designing nonlinearity in maps.
I'd divide nonlinear into two general categories: "multiple intended routes" and "no intended routes." And I would also divide this on a micro and macro scale.
When you're designing for multiple intended routes, you're basically just saying "hey, I have more than one way that I intentionally designed the level to be completed in," this will likely look like something with branching paths. For an example in base game, think the section in Chapter 3 where you have to clean up the mess. You have three intended routes you can take. You may also think of rooms where they have strawberries where you have to take a unique route through them in order to obtain.
When there is no intended solution, you're functionally designing a jungle gym. You aren't designing a problem and solution, instead you're designing a problem and the tools the player is given to solve said problem. In base-game, think of the Old Site Badeline Chaser sections where you have to run around and collect keys while being chased. Or if you want an example from SJ, the entirety of EAT GIRL. It's actually possible to do this with puzzles too, but it makes people, like, really uncomfortable (take that how you will, I'm not even referencing modded celeste with this one).
When I'm dividing macro and micro, it's a question of scale. Micro is referring to individual movements, while Macro is referring to the room layout as a whole.
So, let's start with micro, the individual movements. As that's the style of nonlinearity I'm more comfortable with. Generally speaking, my main philosophy on my level design is that I don't really mind if the player takes an alternate route so long as the alternate route is similar difficulty to the intended route. That's how Floating Oil Rig has so many variations on its clears despite being GM+. So, this is a sort of "there's an intended solution, but there's no expectation on the player for them to follow through on it." And this wasn't really something intentionally designed for. The thing I really like about corner tech is that it forces you to reexamine your relationship with spikes and what sorts of things you legally can jump on, so it just naturally resulted in a map that with a lot of minor variations on the solutions for each room. To be clear, this is still a very linear map.
I think the reason this style of mapping isn't as common in higher levels of Celeste is because it's not super compatible with wavedashing, which is what a lot of popular tech is based around. Going nyoom isn't exactly conducive to thoughtful gameplay, since that type of gameplay generally works best when you to slow down and think about what's happening and can see everything kn screen, which doesn't work very well for super fast paced shit. I think in general, raw speed has diminishing returns in terms of garnering an emotional response from the player. It's much more effective to have a room that is constantly slowing down and speeding up in order to get the most out of the nyooms.
When it comes to widening the scope beyond instant-per-instant gameplay, the idea is just to give the player a problem and enough tools to solve the problem. With the bullet hell map, I was just viewing the rooms as jungle gyms with systems that the player needs to figure out for themselves how to navigate. So, the rooms air on the side of being long, which turns the entire map into a bit of a routing puzzle with open-ended answers, that will probably vary from player to player. The player has to figure out what route is the most consistant for them.
I also want mention that a great way to make routing puzzles is to have rooms where you go through the same areas and use the samw things multiple times but for different purposes.
Like, in this room, there's this open area that you visit three times. The gravity spring there is used twice, as is the dream block. So, the purpose of the elements placed in the level isn't static, which lets you get the most out of each thing placed.
Before jumping into a level, I like to make a room in debug or as a throwaway map where I just experiment with different combinations of stuff. Like, looking for synergies between mechanics. For example, Theo + Gravity Helper is a really strong combo because now the player has to consider where they are, where Theo is, the player's gravity, and Theo's gravity. From there, I know that I want dream smuggling to be part of the map, so I decide to have Gravity Helper Dream Blocks and regular Dream Blocks since that adds more stuff for the player to keep track of. I want to stick exclusively to gravity toggle in this map because toggle gravity is the most open ended variation of gravity helper. I'm using double dash so that I can force the player to dream hyper without needing obvious telegraphs and double dash generally makes handling Theo feel a lot better because it offsets the mobility debuff you naturally get from Theo. Like, a lot of thought can go into just picking which mechanics you want to use, and you can get a lot of value out of stopping and thinking about how those mechanics will interact and what they bring out in one another. I play a lot rpgs, so I think the ideas of min maxing and customization are heavily impacting how I view level design, but also, yknow, just putting a lot of thought into why you're doing something does tend to make the end result more thoughtfully put together.
And yeah, the concept of making rooms loop and choosing mechanics does apply at literally every difficulty level. Like, here's a room from a GM+ difficulty puzzle map.
Like with before, it's a routing puzzle. And you go in a loop. The puzzle is to use the camera offset trigger to perform a spinner deload and then perform an off-screen ultra before the spinners load. The idea behind this map is that you're navigating a space where nothing is really meant to survive, so you're just blatantly glitching through reality. Like, you have to softlocke the game to progress through the level at one point. And that was a decision made around the idea of getting across a certain atmosphere. I want the map to be existentially terrifying. Like, you're lost in a maze without an exit. So, the unintuitive solutions and puzzles are in favor to that concept. But it also results in what is definitely not what you usually see from GM+ gameplay.
Here's another GM+ example.
I've had to edit the layout of this section a little to enforce the solution, but you can see how the player has to loop around and reuse elements in different contexts. That's more or less the main thing about how I go about designing rooms without obvious paths in mind.
I sort of see the GM+ gameplay as tech puzzles. Where the player has to consider their wide catalogue of tech and determine which tech is appropriate for the situation. So, simply making the required tech not obvious can go a long way. This can also probably be applied to lower difficulties, but, like, where else are you going to force the player to steer a superdash variant demo through some spinners with the ultra boost and then force the player to corner jump immediately off of that? But, like, there's definitely room for routing puzzles where the player has to choose between super and demo hyper, for example.
Not all of this is nonlinear. Like, the three maps I gave videos for are technically linear in the sense that there's really only one solution and potentially cheese. But these are all nonscripted. Or at least, maps that are asking for more than just inputs from the player.
Anyways, the more macro heavy form of nonlinearity is a different beast all together. Personally, I'd recommend drawing a map of your level before you open Lonn or Ahorn. I mean, like, pencil and paper. That way you can know ahead of time where to plan for connections between rooms. Having the player zig zag through the level is also really helpful. Like, there's a lot of power you can get by just moving left. It can also really help to think of a small amount of lore. Not in the sense of creating a backstory or writing anything. More in the sense of "this is a castle. this section of the level is the exterior of the castle. this section is the barracks. this section is the hallways. this section is the courtyard. this is the ball room. etc." Thinking of that early can do a lot to not only assist with a nonlinear level, but it can also make individual rooms feel more memorable, it can encourage more diverse gameplay between rooms, and it can make your map more immersive.
You can also totally apply the concept of loops on a macro scale. They're very satisfying to pull off. The first map that comes to mind for doing the loop is Fortress Fall (SJ), which has a looping section. I know Switchtube Vista also did this, but I didn't actually notice on my first playthrough. Tho, tbf, when I was playtesting Fortress Fall, I almost accidentally ran in a circle because I got confused. I think loops are at their best when you're revisiting a section under a new premise, which is a lot harder to do on a macro scale.
Plus, loops can absolutely get complicated fast, both on a room to room basis and as a structuring system for your entire map. I think that's a reason why they're not very common.
But yeah. I think my best advice for this stuff kinda just comes down to circles, not making solutions obvious, and some amount of planning.