I'm not sure I follow, presumably if these are using the Keychain API they're doing so via the app, to which you must be logged in, so they are already tracking you and the keychain thing achieves... nothing?
Discord automatically logged me in ON A NEW IPAD, because I signed into the same Apple ID/iCloud account.
and I assume all the other apps from a company can share that info, like Facebook ↔ WhatsApp
Funnily enough, Apple allowed users to delete that data when deleting an app, during an iOS beta, but removed that feature before release, heavily in favor of the spyware and against users.
then it comes back after you sign into the same Apple ID on that device or even a NEW device!
and of course the crap remains on your other devices.
Funnily enough Apple allowed users to delete that shit for a while, during a certain iOS beta, but then backtracked before release.
My personal pet peeve is that a lot of people seem to believe that disabling Allow Apps to Request to Track disables in-app trackers.
Carefully formulated by Apple to give completely the wrong impression (only blocks identifiers like IDFA.)