This is not beta. It's a quality update. Patch tuesday is a security update and all others are quality updates.
So, is it a "stable" update (even though non-preview updates often cause problems)? I find it odd to appear on WU.
This is stable no-security updates are always used in the next patch tuesday adding only security patches I rarely see other changes. And personally I never had any problems with them although I don't always use the latest updates. And I usually wait a few days to see if another update is coming out.
And are there only CU and .NET CU preview and not SSU preview, FU preview, DU preview, etc.? Also, are they usually released on WU like in this case or only on UUP and/or Catalog site?
So far yes and I see no reason for that to change. "Preview" in this context has the purpose of differentiating quality updates from security updates. Servicing Stack is to prepare Windows for new CUs (as far as I know without security updates). Flash currently receives only security updates (no quality updates). And dynamic updates are for preparing Windows setup for in-place upgrades (as far as I know without security updates too).
There is no reason to call it "Preview" if its a final/stable update unless is it's not fully tested and needs testing. We used to have to CUs if you remember the "security" one every 2nd Tuesday and on the 3rd to 4th week another one mostly quality update. Neither of them were called differently. Not to mention again that the .423 CU is (for the first time for me) under the "Optional" updates. And if it was OK why not give it right away and why it's being mentioned there as "Preview"?
Who understands the logic of Microsoft If before it didn't appear under optional updates and now it appears maybe something will change from now on let's wait.