What's the OTB rule regarding "dirty flagging"?

again applies only to classic chess. And classic chess has increment almost allways

There's plenty of tournaments without increment. (especially with two rounds per day, playing with 30 seconds increment too is somewhat tough as it can blow up your schedule if you have even just one really long game)
E.g. the last tournament I played, the Grenke Open which is a huge tournament, didn't have increment so I saw a few instances there where the exact rule I mentioned got invoked.

@Bd4 According to the analysis board, this is a winning position for white with mate in 138 half-moves. Obviously, >99% of chess players wouldn't know how to mate but you could have just taken his pawn and it would be a draw.

Ah, looking at that original post, the side with the pawn lost on time? That's definitely fine then, this is a known winning position for White. And with the pawn so far back it's actually very much doable. In blitz it's tough of course but in classical I'd fancy my chances with the knights there. The winning strategy is block the pawn with the knight and then with king and the other knight push the black king into the corner, then get the second knight and checkmate them before they promote.

Online and offline: having displayed bad „technique“ sometimes you just have to do it in a dirty way. I can’t help, that’s perfectly legit.

Random sample against a FIDE FM:

I don't get what you mean by "dirty" flagging or "unethical" winning on time. If a player's time expires, he loses. Unless of course the exception of insufficient material.
Nothing wrong with winning or losing on time. It is part of the game.