Thanks to Marius, looks like when you interact with a server you get assigned a local id that’s tied to your username/email combo. So no global ID but all your actions on a server are tied to you. It wouldn’t take a lot to link that to your account on your home server.
Overall idk if it’s a big deal. Upvotes and such need to have a user ID tied to them in order to keep things straight. Unless someone wants to update the Lemmy software to start encrypting the GUID on the table an instance owner will always be able to see what you’ve voted on ON THEIR instance. Reddit totally can as well, but they had a reason not to put their users on blast (At least until lately lol).
It’d take some collusion to put that all together across the fediverse.
Someone should write up the feature as described and explain why this particular implementation solves issues people are having as a pull request on the repo. This would be much easier to implement than a custom tagging feature like others are asking for. Like yeah a custom tagging feature would be cool but that’s a ton of work and wouldn’t show up for quite a while.