this is peak GMing

Every table is different but I don't think this would fly at my table. It's a little metagamey to ask unless you have reason to believe they would, so if they were told in character to look away but maybe the players themselves forgot where it's not something the PCs would forget. It feels a little adversarial. Combat is often rough to begin with and imposing disadvantage for no reason other than it punishes them for taking cues from the DM seems like it's stretching it unnecessarily. But every table really is different and it may have been super fun for everyone, I don’t know.

The DM can not metagame, definitionally

Really depends on your definition, but by most online consensus and generally by most definitions I've seen, a DM can indeed metagame. If you personally don't feel that way, that's fine, but a DM is considered a role player in my experience so the following definition holds:
The act of a roleplayer making use of knowledge that they have learned out of character (and which their character does not know) while they are in character

If I have a random shopkeep tail the rogue of the party through the shop, even if their character does not give off rogue vibes, I'm being metagamey. I know they're going to try something so I use that to change the behavior of the shopkeeper. To me and to most other people according to a quick search, that's metagamey. You can feel differently, but I believe that puts you in the minority.

I think it is - in fact - the exakt opposite of it. It's gaslighting and player vs. DM mentality. If I was a player, I would have a serious word with the DM afterwards and when I am the DM, I refrain from such actions altogether.

My response was (is?) tongue and cheek.

Generally, I think it's best for the GM to give the players an accurate, understandable, and consistent view of the world. If the local townsfolk have said The creature up the hill turns people to stone when they gaze upon it! then this is a perfectly reasonable course of action, even if the monster doesn't have a gaze attack, so long as the PCs trust the villagers. The villagers just happen to be wrong in this case.

If the PCs have a reason to doubt their senses (in D&D/Shadowrun that might be due to a charm, etc; or in Cyberpunk/Shadowrun it could be cyberpsychosis) then it's also pretty reasonable.

Can the GM do it once to troll the players? Sure.

Would it be annoying if the GM made a habit of it? Probably.

Is it funny for a meme? Yes.