It’s only when you’re already on iOS 26 that you can’t downgrade anymore.

At a certain part they stop signing the OS. At that point because it doesn’t have the cryptographic signature you can’t install it at all and your only choice is something newer.

With point releases or security updates that tends to be pretty fast because they don’t want people downgrading to something vulnerable. When going from iOS X to iOS X + 1 grace period is usually longer before they stop that.

And, of course, if you try and install an iOS 26 backup on the phone that you just moved back to 18… you’re just gonna be on 26 again. So you better make sure you have the right back up.