Mastodon might introduce a "proprietary", home-brew "No quote-toots allowed" flag outside the ActivityPub standard that probably won't be documented anywhere. Just like their "sensitive image" flag, but even more critical.
And if that happens, all Fediverse projects that have a quote-post/quote-boost/quote-tweet/quote-toot/share feature will have to have that flag reverse-engineered from Mastodon's source code and implemented a functionality that greys out the corresponding button under "no quote-toot" posts. And all their instances will have to upgrade to a version that fully supports that flag before Mastodon rolls out that flag. Or else Mastodon will defederate all their instances for being non-compliant.
This will at least affect Friendica and everything that can be traced back to it, including Hubzilla and (streams), as well as Misskey and everything that can be traced back to it, including Firefish whose development is dead and dozens upon dozens of other Forkeys.
If this actually comes, and if it's enforced immediately after release with no grace period, Mastodon could just as well fully be defederated from the rest of the Fediverse due to being incompatible. It'd cause a rift between Mastodon and the non-Mastodon Fediverse anyway because the former would have a requirement for federation which the latter is incapable of fulfilling.