Spec should change: stop requiring Tor relays to support old link versions
In our spec, we have
All new relay implementations of the Tor protocol MUST support backwards-compatible renegotiation
But the v3 link handshake came out in Tor 0.2.3.6-alpha. That's certainly older than any relays. Is it older than any clients that we expect to work?
Step 0 in phasing out old link versions would be to admit in tor-spec that supporting them is not a MUST. I'd say we're ready to do this one any time. We could put v3 as the minimum you MUST implement, or we might pick v4, since it came out in 0.2.4.11-alpha, and that's pretty old now too.
Step 1 in phasing out old link versions would be to actually remove the code from mainline Tor. That's #9476 (moved).