Neel Chauman wrote at Sun Sep 12 19:17:37 UTC 2021:

my updated proposal says that if one dirauth gives a relay the MiddleOnly flag, then it's set for that relay. This is to prevent harm while all (or the majority of) dirauths give the relay that flag.

Imagine one hostile dirauth that votes for this flag on every relay it cannot control. Guessing the result is left as an exercise to the reader :).

(Spoiler: IIUC, this will result in using only attacker-controlled relays as entries and exits, resulting in both-ends control of all external and hidserv circuits).