Months after that post, at least two famous brand middleboxes were found to be incompatible with the finished TLS 1.3 because somebody cut corners as follows:

The specification says: YOU must choose RANDOM numbers otherwise bad things could happen.

[ TLS 1.3 final hides a downgrade signal in those random numbers if you appear to only speak TLS 1.2. The TLS 1.2 specification says nothing about a downgrade signal, so if you recognise the signal that means you wanted TLS 1.3 but the server has been told you wanted TLS 1.2, a downgrade attack is being attempted. Abort! ]

These famous brand middleboxes were too lazy to make random numbers, they'd just take the exact numbers the real server picked and use those. Those are random right? What could go wrong?

The result was that the TLS 1.2 Downgrade signal would get copied into supposedly "fresh" TLS 1.2 connections and trip the abort mechanism.

Just an incompatibility right? Nope. For the years that this idiocy was in those products they weren't actually delivering security, the requirement that you pick RANDOM numbers is there for a good reason - if sophisticated bad guys knew this "bug" was present in the famous brand middleboxes they could definitely have exploited this to snoop connections.