If by “open registration” you mean that anybody with your domain name can just go and create an account with no extra verification, then that’s interesting. Good to know.

Element X requires MAS for registration by design, but login is whatever for now. They’re very adamant about making the app MAS-centric.

Also, open registration will get your instance banned from matrix.org homeserver afaik. Probably not an issue if you don’t plan to federate with them, but worth noting.