Mind you, they did foresee flatscreen displays even if they figured it was still a version of the CRT. But in "Requiem For Methuselah" they did have an actual thin flatscreen display not far removed from what we might see commercially in the next few years.
Much earlier, I'd say, already in "The Corbomite Maneuver
Flat screens were still tiny in "The Conscience of the King
Built into the wall in "Court-Martial
" where we also saw a very large one
(former main viewscreen of "pilot" Enterprise
Later seasons featured flat screens in the transporter room in a couple of episodes.
Please notice that the former main viewscreen had an aspect ratio of 16:9
. WOW! (and there we have an original TOS scene in its original and untrimmed 16:9 ratio