You sure?In my experience it doesn’t overwrite ios 18. Yes it will backup iOS 26 but when I want to revert back to iOS 18, when restoring, at the end of the setup, there’s an option to select the last backup of iOS 18.
Because if that’s true I will backup my iPhone , otherwise I won’t backup it until the RC!
But it’s not that important the backup for me as I have iCloud Photos active and synch it every day.