data sync between macos catalina and iOS 26

According to Apple's Technical Specifications for the latest iPhones (17-series and Air),

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Syncing to a Mac or PC requires:

• macOS Catalina 10.15 or later using the Finder
• macOS High Sierra 10.13 through macOS Mojave 10.14.6 using iTunes 12.8 or later
• Windows 10 or later using iTunes 12.12.10 or later

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In practice, people who are still on High Sierra started reporting mixed results (on these forums) when iOS 17 came out. Some had no problems, others could get things to work (after going to some trouble that shouldn't have been necessary), others might have been out of luck completely (not sure).

I have seen no widespread reports of problems syncing with Catalina or later, using the Finder. So I don't think you are going to run into problems with using a Mac running Catalina to manage an iPhone running iOS 26.

As to when Apple will raise the minimum requirement past Catalina, there's no way to say. It used to be possible to manage iPhones using El Capitan or Sierra, but that changed when iOS 17 or 18 (I forget which) came out.

With respect to iCloud, see System requirements for iCloud - Apple Support. Most forms of synchronization that work with Macs still work with Catalina.

Some things like iCloud+ features (Hide My Email?), and sharing things with other people (not just between your own devices), have a minimum requirement greater than Catalina.