First it isn’t “vaguely” against OPTAH. It is a clear violation of that. And OPTAH is a specific rule; at least in RRoP.

Also from RRoP, the Proper Behavior section has these,
“- Reading a hand for another player at the showdown before it has been placed faceup on the table.

- Telling anyone to turn a hand faceup at the showdown.”

Both appear very applicable to the situation in op.

Problem these days is dealers saying "Turn them up." at showdown.

Is saying turn em up much different than the word showdown though?

This would tilt me so bad idk if I would ever play on this dealer’s down, let alone tip them. This isn’t even a casino procedure, this is a poker procedure.

Not much different, but different.
And the line might be just there, who knows?

Certainly the OP's spot is not a fun one, not really sure why other Players would be suggesting that the pot be split in any manner. Would they agree to split the pot if it was them in the same spot? I've missed out on 3-5 pots in the last year where I mucked a winner or chop and not once did a Player even flinch at accepting the pot.

I had a spot in PLO where I actually tabled two of my cards that were a OESFD OTF but 'missed' .. I didn't catch that I made a boat on the River. When I tossed in my other two cards the Dealer took the two face up and turned them over and mucked all four 'right away'.

We all know there are good and bad Dealers out there and some rooms just can't (or wont) hold the entire staff to the standards that visiting Players may expect. I tell Floors all the time that I just want consistency so when I come to a room, but I also understand that rooms/Floors/Dealers are all different and stuff falls through the cracks.

Dealers are working for tips and a 30 second Showdown takes away opportunity. There are certainly better ways to handle this spot as mentioned above. The tone of the room will determine how much a Dealer should prompt a Player action at any time during the hand. GL

We have some dealers who say turn em up; I don't like it. I always say "showdown". You need to say that, as it indicates to players that all action is complete, and it is time to either table or muck your hand. A dealer saying "turn em up" could technically be considered to directing players to table their hand, rather than table or muck. Almost all angle/trouble situations arise when players expose their hand prior to the dealer announcing showdown, thinking action is complete when it isn't; or thinking some gesture indicated a check or call when it didn't. So the dealer declaring showdown is an important task.