If this is true, this is probably an anti competitive practice and you can report it to regulation authorities in the US and EU, you might want to consult some non profit that deals with open source and free software for legal advice.
Ubuntu asking its flavors to stop using something because it doesn’t is a head scratcher. Flavors regularly use things Ubuntu doesn’t, things you could argue are more intrinsic to an “Ubuntu experience”, like installers, login managers, icon themes etc. Why single out Flatpak?
IMO Canonical wants to make snap like google play, where people sell stuff and they take a 20-30 percent commission
You might want to post a bounty on it on rysolv (or some better platform if you can find one), That might be a "stronger" or more persuasive signal then posting on social media.
generally speaking adding code to the main code base requires a maintenance overhead maintainers might not be willing to accept, Also there could be disagreements about important design decisions and lemmy devs might not want certain features, it's really hard to know in advance what works best so third party extensions can help a project switch from a mode of "intelligent design" to "evolution", people just try stuff and the best stuff stays and become popular.
Also advanced features might clutter the UX , but power users could be the ones driving the popularity of the platform so that's one way to attract them.
See here. The graph for six month active users is a little glitchy (I think because lemmy.ml was listed twice under two different URL).
There does seem to be very small growth in 6 month active users, not as fast as a few other fediverse platforms (such as friendica and writefreely) . but i got my fingers crossed that third party lemmy tools will create some really compelling features and help push the adoption of lemmy (I think addons can enhance open source software, like how firefox addons helped firefox adoptions).
Things are probably not as black and white as that, The US have fined corporations before for anti competitive practices.