I know what you mean! I remember when I upgraded to iOS 10 (?) and it stopped being recognized by iTunes. My computer was too old the run the latest iTunes. I had to use a workplace computer to downgrade; I was really nervous, because I upgraded late, so the window for Apple to sign the old IPSW was closing fast. I went to work on the weekend just so I can downgrade ASAP. I wished they would have given me a warning! I updated via iTunes, so I thought that it would at least tell me it won't work with the iOS I was installing.
Anyway, I can't figure out where you found this information. When I type "iOS 16 System Requirements" in Apple's search box, I get only a new feature list. I'd like to keep this page bookmarked for future udpates.