My mother has a 2008 iMac currently running El Capitan. Every now and then it pops up the notification suggesting that she upgrade to Mojave. But if you actually try to download Mojave from the App Store, it says it's not compatible with this Mac (according to Mac OS X Compatibility Guide, El Capitan is the latest version for her Mac). Why is it suggesting an upgrade that isn't actually possible, and how can I stop the notifications?
Many web sites, including Is it possible to STOP the 'Upgrade to macOS High Sierra' message that keeps stalking me?, say to remove
/Library/Bundles/OSXNotification.bundle. But there's no
/Library/Bundles folder on her Mac in the first place.