I know this is late because I wasn't here for whatever nonsense happened again apparently last week but I want to give in my own two cents from the perspective of someone who actually ships it.
The problem, as far as I've found, isn't that people dislike Sonadow. Honestly, there are quite a few of us who don't care about that. I have had multiple friends who don't care for it who I'm on perfectly good terms with, and I've talked out things with people about it with absolutely no bile coming in between us. I'm sure there's some overzealous Sonadow fans, but then again, there are overzealous fans of any popular ship. I could give you multiple horror stories I have personally experienced and have seen happen with Sonamy fans (it's actually the reason I don't doubt there are these negative Sonadow fans in the first place).
However, the reason why a lot of us get upset at this new fad at telling us to tag Sonadow-related things is because, quite frankly, there is no legitimate reason people can give us for doing it. Here's a popcorn list of reasons why it bothers me and other people that we're being asked to tag Sonadow:
No other ship that I have seen has ever been asked to tag it. I've been here for four years. None of my friends have ever come to me telling me that they've needed to tag their ships. Not even Sonamy, which is even more ubiquitous than Sonadow is.
Most people have XKit now, so if a post even mentions Sonadow you can use XKit to block it just fine and it will never appear on your dashboard. Moreover, you can use XKit to block entire threads from appearing on your dashboard, too. There's no excuse not to have XKit, especially since it has a mobile interface too. (Except if you're on your school computer, but honestly going on Tumblr on a school computer is an inherent risk anyway what with how much NSFW content pops up on this site.)
It begs the question of if you hate Sonadow so violently, why are you following people who ship it in the first place? There's an unfollow button for a reason, you know. If those fans get offended you unfollowed them then they weren't really worth your time to begin with. None of the reasonable side of us are going to cry buckets because you would rather not see our ship on your dash.
This isn't the case of it being a trigger, either. To claim it's a trigger would be trivializing triggers. (Not to mention it's severe emotional blackmail which is incredibly disgusting.) Trust me, a lot of us are conscientious about triggers, too; I tag every single one that comes up on any of my blogs, even the ones that center around those triggers as plot points.
I have not, personally, seen any evidence of Sonadow fans going on a rampage against those who dislike it. That's not to say it hasn't happened, but I just have not seen it. Yet, I have seen and experienced plenty of people who dislike Sonadow making us out to be foreign aliens who are The Enemy and are trying to convert them to a ship. I have had someone entirely kick me out of their Zone because our Sonic and Shadow interactions got mistaken for Sonadow once and he knew I liked the ship. I have had people, during multiple instances, talk about me where they thought I couldn't see them because god forbid I like a ship that they don't. I have had people tell others to not talk to me and effectively get rid of any chance of me making friends...over a fictional ship. Also, try going through the #sonadow tag. If you go far enough back, you will find plenty of posts in there of those either making fun of the ship, or making fun of people who like it. We have seen it multiple times in our own tag.
It's a ship and personally I don't see why ships are worth this much vehement hate and telling people what to do, to control them and to get them to be scared off enough from playing a ship on Tumblr as much as possible. Well, if your intention is to get me to not want to be here any longer because of a ship, it has to some degree worked. I'm not in control of your dashboard and I hate to sound demeaning, but...this is my blog. These are my blogs. I should feel free to post harmless things (yes, like Sonadow) that I like there without fearing that people will up and abandon me and talk behind my back over it. And yet I don't. (This isn't the only thing that's making me feel that way, either, but that's a discussion for another day.)
Listen, all I'm saying is that I don't make several passive aggressive posts about other ships that I dislike (and believe me, there are quite a few) and I don't shame people for liking a ship, nor do I blow them talking about ships to me out of proportion by saying they're shoving it down my throat. It's a ship, not a swear word. You can say that you dislike it, even make fun of it for all I care, but the negative treatment of Sonadow fans here is pretty saddening in my opinion. We're humans here with feelings just like you.