I for the life of me can't figure out how to bring up the sort and filter sidebar on ao3. I can get it through the hyperlink you provided in your sort and filter post, but I can't bring it up on ao3 otherwise. I can't seem to find tutorials on it, though.
I think I might know what the problem is.
When you go onto ao3 and just a simple search for a ship, it pulls up a different set of results than the link we provided, which is a link of all sheith works on ao3-- here is the caveat.
You need to select the tag
I’ll break it down a little
I searched “shiro/keith” in the search bar, and this is what came up for me: ao3 filters every fic on the sight and puts them together, so long as they have Shiro and Keith somewhere in the tags. These are not all exclusively sheith fics.
At this point, as you pointed out and I forgot about, the Sort and Filter sidebar is not present. Instead there is a smaller button that just says Edit Your Search. Ignore that.
What you need to do is click on the ship tag itself, as highlighted above.
Clicking on “Keith/Shiro (Voltron)” takes you to all of the works in that specific tag.
As you can see, it looks different. Once you are on this page, the Sort and Filter bar presents itself on the right sidebar, and you’ve got free run of the place.