So we get nice things like subqueries, which using them now was somewhat dependant on what the cluster was running.
MySQL 5 has been production release since October 2005 [1], I think we've been supporting it long enough.
I can't imagine too many people are still running it...?
[1] http://en.wikipedia.org/wiki/MySQL#Product_history
Version: 1.22.0
Severity: enhancement