Title says it all, I can't find any way to set world-specific permissions. The basic use case is that I let long-time members fly in the overworld, but I don't want them to in the end or nether.

This can already be achieved using contexts, found here: https://github.com/lucko/LuckPerms/wiki/Context

As example, you would use /lp group mvp permission set essentials.fly world=world_end, which would allow the mvp group to use /fly in only the end