You can downgrade an iPhone's firmware IFF Apple is still signing that firmware version for your phone model. They don't make it easy to figure this out...but

You can downgrade an iPhone's firmware IFF Apple is still signing that firmware version for your phone model. They don't make it easy to figure this out...but this site shows you what iOS versions are still being signed: https://ipsw.me/ https://ipsw.me/ You can see Apple is only signing iOS 7.1.2 for the iPhone 4. For the iPhone X (iPhone10,3) they're signing 13.3.1 and one version below it, iOS 13.3. So you could still downgrade to that. In practice, if you're currently on the latest version and you update to a new version as soon as it comes out, you can almost always downgrade back to what you just had. But of course, if you wait too long without downgrading and Apple stops signing the old version you want, you'll be stuck with what you got.