what precisely is the update? a new OS for the MacBookPro?
When I plug my iPhone into my MacBookPro, I get the You Must Update this Computer ... what precisely is the update? A new OS?
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If you are referring to this advisory:
...the update is only required for your computer to communicate with your iPhone. Simply "Click Install to download and install the software so your Mac can update or sync your iOS device."
Following that advice will not update your macOS.