Clarifications, please.
Do you mean there are partitions, one of which is Yosemite? Or do you mean that you don't want them to be able to check information and see 10.10.x?
Or now that I read your post again, you don't want them to see the availability of the update in the App Store? If the latter, good luck. Apple has made a banner touting Yosemite on the Update tab of the App Store. However, it does not trigger a red "update available" badge on the app icon.
Which, what??
On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
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