How could I delete iOS 15 (beta software installed and copied in the backup) from the Backup folder on my Mac?
Long story short:
I want to restore my iPhone without iOS 15 but with all my other files from that backup
iPhone 11 Pro, iOS 15
You can't do that. If you want to revert to the shipping. version of iOS, you can only restore the last backup you made before you installed the beta.
You can not restore a backup made with a newer version of iOS to a device running an older version than that which created it.
Backup from Mac
