My question is, do these user-level applications have to be distributed
along with Emacs, or could they be made to be "explicitly" opt-in by
installing them from ELPA. In-core appears to usually bring a
commitment to maintain a library, and deprecating can take years. If
Emacs 29 lays the technical foundations, the low-level API for
treesitter to work, we can have packages on ELPA experiment with the
higher-level abstractions. Whatever is the most successful approach,
can be added to Emacs later on.