Adding a redirect should make Bitbucket redirect HTTP/SSH automatically (BB-4496)
When deleting a repository, you can setup a redirect. This just results in a link being shown when the old repository is visited in a browser.
It would be nice if a real HTTP redirect was issued when a Mercurial client tries to access the old URL. For SSH there's no support for sending redirect information back to the client, but I guess you could maintain a symlink or similar on your server so that old SSH URLs remain valid.