Apple digitally signs all releases of iOS. If they stop signing it, then you can't downgrade to it. Apple's software checks this before any upgrade/downgrade procedure.
Apple does this to prevent downgrading - exactly the thing you want to do. Apple prefers to keep users on the latest version as it makes it easier for them. There are ways around this, but unless you have a jailbroken iPhone and SHSH blobs for the version of iOS you want to downgrade to then you can't. That means your iPhone would have already needed to be on a specific version of iOS to get the blobs in order to later downgrade to that version.
If you have no idea what an SHSH blob is you don't have them.
And, as @C DM mentioned, iOS 13 was never released for the iPhone 12 so it's a moot point any way you cut it.