I will start testing this

I wonder is there any reason why NO_UNDO was not added when drawing the regions in

I doubt that there is a measurable performance difference from doing that, because those regions are drawn only a single time for each view, but it probably also doesn't hurt to add the flag. So I would add it just for consistency. And then I would probably also add the HIDDEN flag, which in this case doesn't have any effect because the regions are empty anyway, but it feels more correct to have it and it might make the code a bit more comprehensible.

Perhaps NO_UNDO is implied anyway when the HIDDEN flag is set, but I guess explicit is better than implicit ;-)
Also maybe this already answers the question above and NO_UNDO is implied for empty regions in general, but I wouldn't count on that and we don't know how ST manages its undo history internally.

For example

REGIONS_INITIALIZE_FLAGS = sublime.HIDDEN | sublime.NO_UNDO 

(haven't tested though)

Seeing decent memory savings on this branch.

This is tested on the typescript-language-server repo with 10 different files opened showing semantic tokens and code lens annotations.

The physical session file doesn't seem to be affected much. Which I guess makes sense because those regions don't survive restart so those are only stored in memory.

Before

After

I haven't noticed any issues with this.