> code that does not compile. Why should I share that?

If you collect test cases for compilers, for example.

> tree-sitter sort of handles that

My worry is that stability of committed ASTs would depend on tree-sitter being stable, and it might be difficult to guarantee that for languages are still in flux. Even most well established languages gain new grammar once every few years, sometimes in backward incompatible ways.

Maybe you meant tree-sitter itself will also be versioned inside this repository?