Might not be reflected in browsing stats, but according to this: https://developer.android.com/about/dashboards/ it's 3.8% of Android users. And that number undercounts non-Google Play Services using Android devices.
Old clients (such as Android) will still be fine, if the servers don't also turn off older versions/options.
When you're handling your own PII and potentially others (contacts, addresses, phone numbers etc) I think having an absolute minimum requirement is beneficial for the Internet as a whole; for example, it'd not be easy to use Windows XP and access the Internet, for good reason, I'd hope (I can't verify that but I doubt browsers still work there).
This is good news all around, forcing people to upgrade is a good idea in the long run.