Did they have anything to say on whether or not they still require (or badly need) an Adobe stylesheet in ePub 3 files?
Our ePub files validate using the official W3C ePubCheck utility. So beyond that simple fact, I’m afraid I cannot comment on what they mean by saying such a thing. 
Granted there may be some combination of settings and content that produce invalid ePub files, but if that were a problem you’d be seeing warnings in nearly every modern reader. Might as well still be a good idea to run ePubCheck yourself, if you haven’t already, on the .epub file in question. (If you use Homebrew, there is an epubcheck package, and if you don’t have Java installed, the adoptopenjdk cask is what I use.)
To reiterate on a comment I made before, version 3.1 is perfectly capable of creating an ePub 2 format file. We didn’t remove the engine, it is merely deprecated. To get this working to your favour, you would need to download 3.0.3 and do the following:
- In 3.0.3, create a temporary project.
- Enter the compile view and switch to ePub 2 or Kindle Legacy.
- Duplicate and edit the built-in “Ebook” format.
- Click the gear button above the pane list and make sure the format is set to both ePub 2 and legacy Mobi, but not ePub 3 or KF8. (Should be how it is set up by default.)
- Change where the format is saved to “My Formats”. This will add the dependency on ePub 2 globally, so all of your projects will be able to select from it again.
All right, you can close 3.0.3 and discard the temporary project. Upon returning to your main project in 3.1, you should now find ePub 2 and Kindle Legacy are available in the main Compile for dropdown again.