jabber.org doesn't accept new registrations since 2013.
If you're looking for a public XMPP server to register an account at there is a (no longer updated) list of Jabber servers at https://xmpp.net/directory.php and a more up to date list at https://list.jabber.at.
You can register an account any public XMPP server at https://conversejs.org (via the chat client in the bottom right).
Incidentally, I run the conversejs.org server which is open to registrations.
If you're looking for a public XMPP server to register an account at there is a (no longer updated) list of Jabber servers at https://xmpp.net/directory.php and a more up to date list at https://list.jabber.at.
You can register an account any public XMPP server at https://conversejs.org (via the chat client in the bottom right).
Incidentally, I run the conversejs.org server which is open to registrations.