Apple Pulls iOS 16.5.1 and macOS 13.4.1 Rapid Security Response Updates Due to Safari Bug

This later article makes it sound like the issue was with websites using UA sniffing:

For instance, after applying the RSR updates on an iOS device, the new user agent containing an "(a)" string is "Mozilla/5.0 (iPhone; CPU iPhone OS 16_5_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.5.2 (a) Mobile/15E148 Safari/604.1," which prevents websites from detecting it as a valid version of Safari, thus displaying browser not supported error messages.

I hope Apple's use of the Rapid Security Response system here was mostly an infrastructure test. I would be miffed to learn that a patch for some zero day was fumbled because Facebook didn't get the decades-old memo not to use UA sniffing for feature detection.