Hey! I recently redid all my sample scenes so this is a bit disconcerting to hear...
Caret: That first issue is due to how unity enters play mode. I've been wanting to change this behaviour for a long time. Basically. in-editor Unity starts counting Time.time up before any visuals can update. So... SuperTextMesh is set to read on start, but by the time Unity actually updates the game view, it's done. If you set the read delay to something slow, you can see it's working just fine. In a real-world scenario, this editor delay doesn't appear to happen when starting the game, loading scenes... but I'd still really like to figure out a way around it someday.
StageScene: I can't seem to reproduce this, I'm seeing text just fine in 2021...
TypingInput: This one, too! It's working fine for me in 2021.
UI & Pixel Snap: Also fine for me... what's going on...?
UI Rect Math: same as Caret.
Underline and strikethru: it looks like in the scene view that the strikethrough is rendering behind text, and this is because the prefab I'm using has the same UI Layer and order as the text. I'll change this for the next update. As for the strikethrough not reading on initial read... now *that* is strange. If I slow down the read delay, the underlines that are meant to happen *will* happen on the characters that are reading out. So... this won't happen in an actual built game, but this shouldn't happen in the first place because of how STM (is supposed to...) execute events. I'll take a look into this, but I think it's the same bug as the Caret issue and won't happen outside of the editor...?
That last one I can fix for sure, I compile Super Text Mesh in a super-old version of Unity, and it looks like the GUI Layer has finally been removed. I don't think I use it for anything so... away it goes.
Anyway, that's about all I've got for tonight. Until I can reproduce those 3 issues I can't really help with them, the parts that are malfunctioning are completely new issues. I hope I can figure out the reading-before-playmode-actually-starts-in-editor issue though, it's caused me some issues in personal projects, despite being superficial.
This isn't the solution to the problem, but I'm curious why people are actually using 2021.1 for development...? Generally it's safer to stick to one of the LTSes. Cause if this really is the cause of some weird Unity behaviour... I can't really fix it and there's no point in it in the long run. But if 2021.1 has a new feature you need, stick with it!