It’s stable indeed, just as stable as the version 2.2 where you’re in. The issue is that if there’s any security update, you won’t get it as expedited as possible. That’s really most of the issue.
Discourse handles something like: master → test-pased → stable. Being test-pased the most stable and updated version of Discourse (that’s why it’s the default option).
It’s your choice to go to 2.4 (test-pased) or stay at 2.3 (stable).