Actually it's even worse than that. I tried yesterday to register on https://lemmy.ml but it let me go through the registration process up to the submit button before returning an error message "Registration Closed".

• It's very annoying to have closed registration in the first place.
• When registration is closed the registration form should definitely not be present and let me fill the whole form.
• The error message should not just say "Registration Closed", there should be some indication of where else I should go to register.

I have to agree here. I went through this same issue with Mastodon when I joined a smaller instance "mastodon.lol". It shut down without any apparent notice to me (using a third-party app). I had to start from scratch and mastodon even has an account migration feature.

I can't in good conscience tell a less technical friend "oh go use a smaller instance" when that means they can come across a dead-looking thread or community (that just hasn't been federated to them yet) or suddenly lose their account when their instance disappears with little to no notice.