>Is it “you are not allowed,” or Cupertino isn’t going to bother developing and testing?

It is very firmly "you are not allowed". In fact you're not even allowed to switch back to iOS 18 at all. Only actively signed iOS IPSWs can be installed (barring historical cases where someone had saved signing tickets). You can see the current status at sites like https://ipsw.me and if you're on any iOS 26 supported iDevice currently only 26.3.1 is signed. The last iOS 18 version was 18.6.2 from August of last year. If you go back to the iPhone XS/XR, you'll see they're still updating iOS 18, with 18.7.6 released two weeks ago (March 4), but they've chosen to force anyone who wants security updates to move to iOS 26 instead.

The rollback provisions, granted. But I’m arguing the other stuff requires QC attention Apple may not want to provide to a legacy line. That isn’t not allowing something that can be done. It’s not building something they don’t want to.