You have to start a new service that offers that feature as one of its differentiators, then the competitors might add it (back) to catch up.
any reason why Forgejo/Codeberg couldn't/wouldn't adopt this?
The only git-notes related issue I found is https://codeberg.org/forgejo/forgejo/issues/6385. So, probably because nobody has raised it.
> Here is a plea for all forges: make code review metadata available offline, inside git.
I think this will fall on deaf ears as far as commercial forges like GitHub go, since as you yourself observe:
> But much of the value of git repos ends up locked into forges, like GitHub.
For-profit enterprises are not generally excited about commoditising their own value-add. This is not a jab at GitHub -- I think GitHub do everything right (offer a great service, a very generous free tier, and make it possible to extract all your data via API if you want to shift providers). It's just the nature of any commercial operation.