To support decentralization and spread, should lemmy.world close registration at some point to prevent a performance overload due to too many users? Of course, if registration is disabled, there could be a hint placed somewhere near that from other instances you can interact with content on lemmy.world just like you had registered on it. There could be a link to join-lemmys instance overview.
I’m in favor of simplifying the signup process with auto-assigning an instance. [Edit: For users coming from https://join-lemmy.org/]
For people who start using the fediverse or lemmy, the concept of federated instances is hard to understand. It also does not matter that much at this part of their journey. How about randomly assigning new users to instances which are open? This could also help with load balancing between instances.
The idea is to make entry as quick and easy as possible. Once users familiarized themselves with content and communities, they can reevaluate their ‘decision’ which instance they want to make their home. At this point, they have a better idea what this is all about.
Choosing an instance right from the start should still be possible, just not be the default mode. Make it a small link at the bottom ‘advanced mode’ or whatever, just don’t scare or burden newcomers with unecessary complexity.
To answer the question directly: I think each instance can make that decision for themselves, and open or close registration accordingly. Both is fine.