is there a complete list of what has so far been determined as definitely acceptable system-wise?
Well, there's this thread and my replies to each person.
Aside from that, if your supplement is compatible with every edition of D&D, it's fine. If it's not, ask me, I'll look into whatever specific thing you want to make content for and approve it (likely) or explain why I'm not approving it.
There's also all the quote-tweets of this where various creators have approved you to play around with their IP. Those are all pre-approved for submission. https://twitter.com/ForestedDepth/status/1496480906797850626
Sorry for doing things in such a haphazard way, but there's too many OSR systems to list them all. It's a beautiful diversity, but that brings certain burdens with it.