We should have a way to delete LFS files that are not in current repo, as articulated in this github issue.
Of course this is not Bitbucket feature per se, but judging from the discussion on the issue above, github LFS maintainers don't object to the idea, and are willing to have an extension to the protocol ( that bitbucket cloud can take on to implement on their server side).
The question is, who is willing to propose one. And I believe that it can only come from the git hosting cloud providers, like Bitbucket or git.
It would be good if Bitbucket can take charge on this issue, and hash out such a protocol and implement it on the server side, so that such a command can be implemented.