My first instinct says I might have found a fold here as this river card is just the worst card possible...so yeah I get your concerns that the call could be -EV in the long run.

So let's see his line to the turn could be: [combos]
Value beats us by river: Two pair like T9s (rather s only pre?!) [4], QT [9], Q9s [4] (as he rather wouldn't open Q9o pre?!), some of his sets even though you're right and he will play them fast probably [3] -> total [20]
Semi-Bluffs beats us by river: JT not always taking this line (50%) [4], AJss [1] , 78ss [1] and strong A-high flushdraws [7]
But he might not bet his fluhes even..let's say he only bets 15% of them -> [4+1,5]
Total Combos that beat us: [25,5]

Semi-Bluffs we beat: AKdd and possibly some AK in general [1+ 4.5 (half of the AK)], AJdd and possibly some AJ in general [1+ 4.5 (half of the AJ)], JJ [3] -> total [14 combos] but he should probably check these hands often instead of C-betting so let's take only half of them [7 combos]
Value we beat by river: J8ss (should not open J8o pre?!) straight [3], AA [6], KK [3], AQ [12], KQ [9], QJ [9]
-> total [42 combos]
How often is he turning these into a bluff? I was surprised to see him KQ turn into a bluff. This is probably the crucial point where a river call could get +EV or -EV.
Saying he only puts 5 of his 7 combos with A high and the 3 JJ combos to bluff (hands with weakest showdown value)
-> [8 bluff combos]

So we win 8/33.5 = 24% of the time...
Pot odds say are 1 to 2.1 so we need to win 32% of the time.

So well..this might be a fold. But to be honest during analysis I switched some things around. And when we end up with 4 more bluff combos so 12 instead of 8 it's break even. If he's a player with a very high C-bet he will have more AK and AJ bluffs in his range and it could become a good call. Similarly with strong hands turning into bluffs, if he's doing that super frequently it becomes a good call.

-> Takeaway for me: The splitting point is how wide we predict villain to turn his medium hands into bluffs on this river and how often he ends up holding AK/AJ on the river. As a thinking player he should then turn these into a bluff because it's a big scarecard and he is at the bottom of his range.

As I tried to model a standard player (with my guesses) I would have to fold after this analysis against unknown. Also I think the standard player pool rather hesitates to turn value hands into bluffs at low stakes.

BUT: Against your opponent who turned KQ into a bluff this would mean he also should turn QJ into a bluff. Even if he only does it 50% of the time with these hands this is 9 extra bluffs combos which makes it a good call to this specific player, after all.

Let me know if you find any dislogical conclusions / thoughts / errors in this attempt ;)

How I see it, value he has TT,99,QQ (discounted) 9Ts and J9s That's about it, bit of a stretch to give him many Tx combos imo
Bluffs AJdd and pairs turned into bluffs, I would discount Qx,KK,AA turned into bluff at these stakes (not sure if right).
What else do we expect to see here from a reg ?
So I believe Fold max EV
I think his bluff is ambitious but quite good, there are not many hands available to bluff, bit unexpected on nl20

I would fold preflop to be honest, KJo don't play that well OOP, you could make a case by 3betting maybe but I would just fold.

As played, I would also fold, people just don't bluff enough at this spot at this stakes, I respect villain for taking the shot (was he bluffing or just clicking buttons??), but I don't think the tendencies of the pool justify this call at all.

Hi Eldora, thank you so much for your great answere. It makes so much sense!