The answer is unfortunately no.
Apple is no longer validating the signed IPSW file for iOS 12 (which you don't have since it is stock). They only validate it while that is the current version and for about a week after a release of the next version.
If you jail broke the phone, then possibly you could do it with a backup of the phone in iOS 12 (after jail break), and if you can find iOS 12 IPSW file somewhere on the net.
Here is an example of the revert back to previous release:
If you had used iTunes to upgrade your iPhone to iOS 14.3 last month, there is a file in your folders (C:\users\<login>\AppData\Roaming\Apple Computer\iTunes\iPhone Software Update) named something like iPhone_4.0_64bit_14.3_18B123_Restore.ipsw
If you today heard iPhone to iOS 14.4 is available and wanted to try it but have the option to go back to 14.3, there is some prep work needed.
You need to make a backup of the iPhone while in 14.3.
You MUST have a backup of the device in the older operating system BEFORE upgrading or lose all your data.
This is because an older operating system cannot restore from a newer operating system backup.
This backup needs to be copied from the usual location to a location (C:\users\<login>\AppData\Roaming\Apple Computer\MobileSync\Backup\<Hex ID of the device>) where it will not be overwritten by the upgrade. You can determine which device is which by looking at the Backups in iTunes in Edit->Preferences, Devices Tab. The date on the backup in iTunes will match the date on the Hex ID of the folder. Copy the folder to a new location (make a Save folder in the Backup folder and copy it there)
Now you can start the upgrade to 14.4, then the file iPhone_4.0_64bit_14.3_18B123_Restore.ipsw is automatically deleted (in your Recycle Bin) and a new file will be put there named something like iPhone_4.0_64bit_14.4_18B12_Restore.ipsw
Now if you hated iOS 14.4, you can revert back to iOS 14.3 (in the first 5-7 days after release of 14.4) by:
- Delete the iOS 14.4 IPSW file
- Restore the iOS 14.3 IPSW file
- Delete the iOS 14.4 backup of your iPhone
- Restore the iOS 14.3 backup of your iPhone
- Force the iPhone to go into Recovery mode
- Hold Shift and click the Restore button in iTunes.
- Select the iOS 14.3 IPSW file
- Restore the backup.
Ugly process.