Once an editor stops being considered a beginner, we could send an email like:

Hi!

You have fulfilled (list beginner conditions here) and you are no longer considered a beginner to MusicBrainz. This means your beginner restrictions have been lifted, and you can now vote on edits by other editors. See our guide to voting, and some voting ideas to get you started! (add any other restrictions here if we add some, like all their edits being votable)

Additionally, here's some other suggestions of what to do next: (remove anything we know do not apply) 

• You might want to try some userscripts to make editing simpler and faster, if you haven't yet. See our list of known userscripts.
• You can try entering extra information for releases in the relationship editor. See more about this here.
• You can try tracking your music listening on ListenBrainz, or writing some music reviews.
• You can try adding information about genre and mood to releases, recordings and more.
• ...

We're very happy to share this milestone with you and (something else nice).

This just feels like a nice thing to do, and might actually help more people start voting, which we don't really introduce to editors much at the moment.