My uninformed guess is that Atlassian will rename the plugin to Bitbucket Data Center Integration and stop testing it against Bitbucket Server. Then the compatibility can break without notice, as a side effect of some other change.
I hope most companies will switch to a supported product, rather than keep using Bitbucket Server after Atlassian stops providing security fixes. But if some companies have to keep using Bitbucket Server, and the Bitbucket Server Integration plugin becomes incompatible with that, then those companies can submit pull requests to restore the compatibility, or to add any missing features to the Bitbucket Branch Source plugin. (Although the most requested feature seems to be cloning from a mirror, which requires a Bitbucket Data Center license anyway.) If the PRs are rejected, then either plugin can be forked.
mhenschke_atlassian - Is there any status on this ticket? I noticed you logged it for someone from a community post so you don't have a lot of information here. This is something my team strongly desires as we use multibranch pipelines heavily and would really love the ability to auto-find new tags. I have reproducers setup and ready to provide more detail if you'd like that information (e.g. webhook trigger info from Bitbucket to Jenkins upon tag creation).