[esafak]

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.

[xeonmc]

any reason why Forgejo/Codeberg couldn't/wouldn't adopt this?

[bicolao]

The only git-notes related issue I found is https://codeberg.org/forgejo/forgejo/issues/6385. So, probably because nobody has raised it.

[saghm]

Seems like a chicken-and-egg problem. Not enough people know about them because they aren't supported by most providers, and because people don't know about them, there's no pressure for providers to add support for them.