I could be wrong, but as I recall, the only requirement is that you need to be running at least 10.15 Catalina to be able to back up an iphone via the finder on a Mac, or you need the Apple devices app if you’re running Windows.
I don’t believe you need MacOS 26 to backup iOS 26. That would screw over a lot of people with older hardware or people that didn’t have Macs.