I suppose it depends on your definition of "stable". All software has some bugs in it so it comes down to which bugs affect your environment.
Given that Splunk 18.104.22.168 is out, I'd say the latest version of 8.1.x should be acceptable. Consult the Release Notes of that and later versions to see if any known or fixed issues are significant to you.
Be sure to test on your non-prod environment, too.
--- If this reply helps you, Karma would be appreciated.