5586502 created the issue

This is in the line of tickets #8488, #8528 and #8537, but not as urgent as the key exchange algorithms.

Standard procedure with hmac-sha-512 is to Encrypt-and-Mac, which might lead to some side channel attacks, according to https://stribika.github.io/2015/01/04/secure-secure-shell.html

Encrypt-then-Mac should fix that. Personally I only care about the 512-bit version of that MAC, but if it is easy to add, hmac-sha2-256-etm@openssh.com can be included as well for compatibility.