Just FYI, while GateKeeper updates are still available for Yosemite (10.10), there's no XProtect update available for Yosemite anymore (it's simply no longer part of the update catalog - not even an old/outdated version!).Just to be clear, XProtect and MRT updates are still provided to OS X El Capitan [10.11] and above (although there appear to be some enhancements to the Catalina versions). They are normally updated automatically if you have enabled that in System Preferences and can be manually updated with a terminal command or SilentKnight.
So your old machine/ setup might just be stuck to the last update it has received (whatever version this is at) and then can't update from that anymore. If you're lucky, it's stuck with 2103 (the last version that was available). If you're unlucky, you're stuck with 2099 or <insert-some-random-number-that-is-smaller-than-2103>.
As a workaround, one can fetch the El Captain (10.11) update catalog manually and extract the direct download link to a more recent (latest?) XProtect update. The downloaded pkg will install fine on Yosemite, too. I'm not 100% sure it works properly, but it can't be worse than being stuck with an outdated version, I guess. A virus test file is still properly triggered at least.
Just a heads-up for people with unsupported versions of OS X. It probably also applies to earlier versions?