My PoC has matured into something quite usable.  The maturing part
included making it robust against mismatched or missing
`*-change-functions` calls, which it tries to detect (the problem
is then passed on to the client in the form of a change with an
unknown "before" content).
I also wrote a test which makes random changes and verifies that
the clients receive correct descriptions.

I currently use it only for Eglot and diff-mode, but I'm quite happy
with it.  For Eglot, it nicely packs up consecutive changes (like
consecutive `self-insert-command`s) into a single change yet keeps
changes to different parts of the buffer nicely separate.

The API is still about the same as before except that
`track-changes-register` now takes 3 options:

- `immediate` to control when the presence of changes is signaled
  (default to use `funcall-later` but `immediate` makes it use `funcall`
  so there is no delay).
- `disjoint` to prevent changes to different parts of the buffer from
  being combined into too large a change change.
- `nobefore` which indicates that the client doesn't actually need the
  `before` contents, so will only get the length thereof (like
  `after-change-functions` does).

I'm proposing we include it into `master`.
I have pushed the patch to `scratch/track-changes` (and attached it below).


        Stefan