Directory authorities should stop listing Tor <= 0.2.1 -- maybe more
We've not been doing security updates for 0.2.1 in a while. It looks like there are still a handful of such nodes in the directories, though. They seem to make up about half a percent of the network bandwidth.
Other versions that we allow include 0.2.2.21-alpha onward. But right now, versions before 0.2.2.35 add only another half-a-percent to the bandwidth.
If we count routers and not bandwidth, omitting these versions would remove a bigger fraction: 1.5% of the routers from 0.2.1, and 2.5% with 0.2.2 versions <= 0.2.2.34.
If we define 0.2.2.35 as our minimal 0.2.2 release, we should probably also reject 0.2.3 versions before 0.2.3.10-alpha. They're a miniscule fraction of the network too.