Hmmm I would like to add my opinion here too

.
I think all of the elements of "Why did I get a red thumb for conceding a victory ?" are now in this thread. So... basically everything I'm going to say has been said already. I'm just gathering things together.
First, we're, indeed, playing actual boardgames, even if it's online. With different boardgames comes different "cultures". In a go game, it's really common to conceed the victory as soon as both players realized it's a one-way game. Keep playing to reach a score of 50 against -60 is nonsense
for this game. Though, it might be pedagogical (somehow) to at least play more moves to see how it would have turned out in some areas.
Let's take another example with an actual BGA game : Quoridor. I think everyone will agree when I say that often, around mid-game, you already know who'll win. Then where is the fun, or even the pedagogical part in spending the rest of the game to place walls and move a pawn, when you know you lost ?
Now, let's take... hmmm Troyes, or Seasons. Both the games are played in different parts. Both are games in which what you played in the very beginning will/can affect the very end of a game. And it take some time to set it up. Thus, you can never really say what'll happen until the very end of the game (like... the last 2 rounds for seasons for example). Then, conceding is rude, and also kind of useless, unless you're in a hurry.
I mean... can you really imagine playing a Monopoly game to the very end when you got nothing left but 1 000$ and your opponent owns 3/4 of the board, and your chances of winning a close to 0% but actually lose for good will take an other hour of playing ?
So... it's not only a matter of abstract game or not (I can't really see the logic in conceding the victory in Gomoku for example... can you

?). Or even multiplayer game or not. If I'm getting raped by Arcano Leeches, Frigrim, Fairies, Titus, Kairn, or whatsoever, and i can't even develop my game, I don't care if we're 2 or 4 around the board. If it becomes impossible to win, then I'll concede the victory.
Now there's an important point. What I said earlier is not even objective. Everyone might feel it in a different way.
Then (cause it's kind of the point of this thread) there's a simple solution not to get a red thumb for conceeding.
And it's called... COMMUNICATION ! Yeah I read that word on the internet once...
Since everyone is different and you can't get to know everyone... just ask.
"- Ok I lost, is it ok if I concede the victory ?"
It's as simple as that. Some people will agree and thanks you. Some won't... then do whatever you want but you know the consequences. Some won't answer...
Well, to me those one are not worth playing with but that's only my point of view =). Again, if you decide to concede to someon who didn't answer you, I think you can expect to ged a red thumb...
Like it was said before... conceding is often see as "leaving" for a lot of people. So it's normal if it feels like a rage quit if you just concede the victory without saying anything. Even though it's a regular way to win

.
I hope this helped.