What generates this message to update?
Solution 1:
Not a definitive answer, but anecdotally…
It's happened on every Mac here that is linked to an iOS device, on first connect after 14.1/14.2 [we held off 14 for a while]
Why, idk, but yes, something changed [or is about to change once you do update to 14] in the phone/pad that needs a macOS/iTunes update.
No Catalina here, but his happened on both High Sierra & Mojave Macs. I had assumed, prior to your question, the it was because we weren't up to date on macOS itself, but it seems not to be the case.