> The version info displayed in `C-h o` comes from the NEWS files.
> It's indeed approximate but I've found it surprisingly usable so far.

Hm, I couldn't find any evidence of it using it for functions, for
example `seq-first` which according to NEWS has been added in 27.1.

> Whichever solution you end up, I hope it can be used for `C-h o`
> as well.

I could imagine such a database to be included with Emacs as part of a
stable release.