Something else to consider, but that I've not figured out a great way to poll yet, is the user representation of those on out-dated installs.
The largest instances in the network tend to have administrators who are aware of updates, and apply them regularly (especially when it's security related) either because they care, because their users ask them to, or because they're on managed providers that handle it for them.
Sorted by monthly active users:
10 of 10
19 of 20
46 of 50
92 of 100
Are all patched.
@vmstan
A useful feature would be for Admins to automatically block instances below a security version level. Might encourage recalcitrant Admins to catch up.
Paging @Gargron