Comment by ekr____

Well, this is basically what we do, except that we try to negotiate to the highest version during the period before the flag day. This is far more practical for three reasons:

1. You actually get benefit during the transition period because you get to use the new version.

2. You get to test the new version at scale, which often reveals issues, as it did with TLS 1.3. It also makes it much easier to measure deployment because you can see what is actually negotiated.

3. Generally, implementations are very risk averse and so aren't willing to disable older versions until there is basically universal deployment, so it takes the pressure off of this decision.