I could have sworn like at the beginning they weren't always open.
I also somewhat remember Sergals being a closed species at first, or at least that there were canon rules to creation, but I got the impression nobody gave a fuck because the creator was Japanese and didn't speak fluent English, so international fans just took the concept and ran with it.Maybe they weren't at first, but whatever the case they seem to be open to the public now.
I think at most fans might have a shitfit if you make a sergal that breaks canon rules? idk