Lemmy

Silently combining communities, which may each have different content policies, is rife with potential for user confusion. Also, there's no guarantee that communities with the same name across servers have the same aims — to use your "piracy" example, c/piracy on one instance may be enthusiasts of literally sailing the seas wearing peg-legs and looting ships, or people who have sarcastically adopted the name for their fandom of those Johnny Depp movies, or something else entirely. Or your desired kind of piracy may take place in communities named differently across servers, whether it's due to someone else registering the community name first, local slang/translation, etc. Ironically, I think what you're asking for is a different type of centralization — centralization of namespace across servers.

The suggestion is interesting, but you may be expecting something out of Lemmy that it is not, as communities are individually hosted and managed. It does sound like there may be potential for Lemmy instances or client apps to allow a user to combine communities, multireddit-style, for their own personal usage. That would be cool and useful.