The zone of impurity is very hard to constrain. Consider an phone with a client app that is out of storage space and can't download an update. Even if you "solve" the problem by forbidding older client versions from connecting, you can't make that cutoff too quick or you'll annoy users. So, you're talking about a lag time of at least weeks on every deploy, which means you've already started multiple new deploys before the old ones drop out of support window. A large system is always in the state of transition.