Since this badge can't be earned by users joining after it was too late, I think it would be fair to remove it from their list. It's somewhat depressing to take a look at something you can't ever achieve.
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
I think it should be left - it's not like it massively increases the clutter of the badge page. What if I want to see a list of all the beta users? Also, how can it be depressing? Surely there are dozens of other unearned badges the non-Beta users can focus their interest on?
I think the answer is: Go to Area 51 and find a proposal you're excited about. Now you can earn that Beta badge.