There’s also the Mutopia project:

https://www.mutopiaproject.org/

They are working to typeset out of copyright works using GNU LilyPond. The scores are particularly beautifully typeset and very easy to read when printed.

Nice resource that's better than nothing, but just want to note that there are numerous engraving errors in the couple scores I looked at that would make them unsuitable for serious study.

Errors meaning wrong notes, stylistic tics, or something in between?

Without a good background in music theory it is very easy to transcribe a piece in a way that may result in the same sounds when played but that is musically nonsense due to the enharmonic nature of many notes. Every flat could have been a sharp just the same, knowing which one to pick requires a lot more knowledge than just what key the piece is in.

I spent many days working out a compromise that at least isn't nonsense most of the time but I know that it will still take much more work to be able to do this reliably. For now my theoretical knowledge isn't good enough though. This is very complex, and the algorithm to do this properly already runs to many pages.

IMSLP is absolutely essential for working musicians! Glad to see it on here.

Love the business model.

Economy of time