Why the need to update MacOS for iOS 14.5 I just updated my iPhone to iOS 14.5... Then my Mac (an old Mac Pro running MacOS 10.14.6 Mojave) put up a dialog saying I needed an update for the Mac to work with the iPhone.  So I updated.
But what did that update do?
Similar Mac &amp; OS, just saw the same notification - being prepared for it i followed through the Learn More… link to Apple KB - If you see &quot;Software update is required to connect to your iOS device&quot; on your iPhone, iPad or iPod touch

When you connect your iPhone, iPad or iPod touch to a Mac with macOS 10.11 or later, you might need to update the software on your Mac in order for it to recognise your iOS device. Learn what to do.
If your iPhone, iPad or iPod touch has a newer version of iOS than is supported by your Mac, you may see this alert: &quot;A software update is required to connect to your iOS device. Would you like to download and install this update now?&quot;

Click Install to download and install the software so your Mac can update or sync your iOS device. If you continue to see this alert, make sure that your software on your Mac and iTunes are up-to-date.

From comments - softwareupdate --history | head  may help figure out what was updated.
I tried it on mine, though I'd also run the new security &amp; Safari updates. There was one other item shown as updated with a timestamp that matches my iOS update, though to version 1.0.0.0 which doesn't seem greatly helpful.
macOS Mojave Security Update 2021-003      10.14.6    27/04/2021, 7:53:05 pm
Safari                                     14.1       27/04/2021, 7:53:05 pm
Mobile Device                              1.0.0.0    27/04/2021, 4:46:37 pm