Why would you want to mandate it? That would kill Fedi faster than anything. Make it an available option that stands the test of time, while other things come and go, and people will use it eventually. (The Fediverse is already over a decade and a half old; almost as old as Facebook.)

Although, there are some proposals in the EU to make it mandatory for social media companies to use open communication protocols like ActivityPub (an exaggeration of their existing obligations under GDPR, which most of them are ignoring already)...