Apple only lets you downgrade to iOS versions that are still signed. Typically previous updates remain signed for ~1 month. You can check which updates are still signed here: https://appledb.dev/

However, you can downgrade to any previous iOS version if a bootrom exploit is found for your device.

For example you can use https://github.com/pwnerblu/surrealra1n to downgrade A12/A13 chip devices using this bootrom exploit: https://github.com/ahmadkamal09999-tech/usbliter8