just writing down my research on this part of the protocol landscape:

to the best of my knowledge, the only one that has seen any real world use and are not theoretical specs nobody follows are GoToSocial's https://docs.gotosocial.org/en/latest/federation/posts/#interaction-policy, potentially with a custom-made extension to expand it to quotes. but it still has downsides (mainly, instances that do not implement the controls will still get the replies federated over just fine from actors they follow).

https://fediversity.site/help/develop/en/Containers is another that can control replies in theory (by Remove'ing from the container collection), however that would break backwards compatibility with every other instance software as it relies on federating the reply to followers as an Add, as opposed to Create (but it'd solve the "replies not federating correctly" problem!)

there's also https://codeberg.org/fediverse/fep/src/branch/main/fep/5624/fep-5624.md, which has mastodon's name on it, but it's still in theoretical spec land instead of being of something implemented (or even just planned) by anyone (as usual for most FEPs)

additionally, there is https://hacktivis.me/git/litepub.social/file/lice.md.html, which seems to be an old and incomplete specification for using object capabilities to do the same, by some pleroma developers. as far as i know this hasn't resulted in anything, but may end up being a useful reference.

(i've also heard vague whispers of mastodon introducing yet another extension (possibly related to the abovementioned FEP) for their quote posting implementation, which may be extended to replies later on, but i do not have any details on that and given how closed-off mastodon's development seems to be moving i doubt anyone will have those details until the last minute)

I'm not really a fan of any of the implementations or proposal I've seen so far, especially the GTS one. I'd say we should wait and see if anything catches on. Should the GTS one gain more adoption I may begrudgingly implement it, but I doubt that will happen given the many open questions and the dubious specification.

Don't get me wrong - I'm all for a good solution to this problem, I just don't see that happening with any of the proposals out there.

Since the creation of this issue, Bonfire has added support for GTS-style reply controls, Wafrn has expressed interest (but haven't implemented them yet), and Mastodon's quote authorization mechanism works pretty much the same way (minus different naming on the authorization stamps). If reply controls are in scope then I think that's the proposal we ought to implement.

Additionally, are Mastodon's quote (and reaction? we'd need to make an extension for that) controls in scope? I've been working on implementing just enough to let Mastodon quote us, but once we implement the stamp infrastructure for that we should be able to reuse it for other types as well.