Villain is a nitty reg. I think flop is a call but on the turn I don't know. Nothing has changed but he still has the nut advantage. Easy to lose here vs QQ+

UTG: 103 BB
CO: 61.9 BB
Hero (BTN): 103.7 BB
SB: 100 BB
BB: 100 BB

SB posts SB 0.5 BB, BB posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has 9 9

fold, fold, Hero raises to 2.5 BB, SB raises to 8 BB, fold, Hero calls 5.5 BB

Flop: (17 BB, 2 players) 4 8 7
SB bets 11.9 BB, Hero calls 11.9 BB

Turn: (40.8 BB, 2 players) 3
SB bets 28.55 BB, fold

Well, if you have 99 in this spot, I think you can also have 88 and 77. If villain is a nitty reg, I don't think he does, so I'm not sure he has the nut advantage. If he is a nitty reg, I'm not sure he has 87s either, and with the odds you're getting on the call pre, you should.

I can understand the thinking, but I think vs this sizing, a call is probably warranted, unless you have any reads that villain very rarely bluffs, and that he probably has an overpair. Without any such reads, I think a call is best, and maybe fold to a third barrel.

If we call we goes to the river with low spr. If the river is a spade or club he can turn his As or Ac hand into a bluff because if I had sets or 2 pairs I would have raised.

can fold flop too

Vs unknown I call otf. Vs this description you can make an exploitable fold and feel good about it

Well, yes to a degree, but you still have to have some hands that you call turn and fold river. If villain has fired two barrels, the odds of a third barrel will roughly be 50%, so you can also win the hand by calling the turn and checking back those times villain checks.

Also, why would you be raising your sets or two pairs here? The only thing you will achieve is folding out villain's bluffs and some overpairs which have very little equity vs those hands. And with the low river SPR, it does mean that even if villain does check it back, you can easily gii.

Maybe 99 is a call ott or not, and I'm sure you could argue either way and not be too badly wrong, but if you make decisions based on some of the arguments you make ("easy to lose vs QQ+" or "he can turn his As or Ac into a bluff"), you will be making wrong decisions. A good way to look over your decisions in situations such as these, where you have a bluff catcher, is to look at it through MDF. This will give you a rough, but fairly decent, idea of what hands you should be folding.

You are right and it has the merit.