Apple lets you run and upgrade the previous version of iOS now, but only for a very short period can you downgrade a major version (and then only to the current version, not some arbitrary past version).
Do not sleep on this chance, turn off automatic updates immediately and downgrade immediately if you can’t be on iOS 16 for the long haul.
If you’re on the fence, go to 15.7 for stability and security updates and turn off automatic updates while you decide on the upgrade. I did this myself, and will wait for new hardware and / or iPadOS 16 to jump to 16 myself.
Now, what is new for iOS 16 is rapid security responses which “update” iOS. These can be rolled back until the next proper update, but that’s not really what I think you’re asking here.
If you need to remove a Rapid Security Response:
- Go to Settings > General > About.
- Tap iOS Version.
- Tap Remove Security Update.
You can reinstall the Rapid Security Response later, or wait for it to
be installed permanently as part of a standard software update.