It is worth to see the terrible bind the black king is in. He can't move, of he will allow the white knight to escape. The only way to break it is by Kd8, Be6, Kd7, to threaten it with the rook. Plus another tempo to take the knight, and another to relocate the rook... a lot of moves! Black here has to take advantage of his well positioned rook (yes the h8 rook!) and maybe try h5. White can't move the bishop to h2 because of the fork on f2, nor play f3 or Ne3 will be very strong, so it has to go h3 almost forcedly. Black's knight can't go to E5 or Rxd4, but can play h5 anyway. White will have to accept the exchange, ending with tripled pawns.

Black doesn't consider h5 as a possible plan... I wonder if it is at all!

Well, this allows the nasty Ne3, but true is that now it is less harmful. Still, I wonder if h3 wouldn't have been better (BxB, hxN).

No, the line you suggest is not better. BxB, hxN, Bf4 wins the exchange back with advantage.

Another interesting way for white to proceed could have been c3. Consider that after dxc3, bxc3, white begins to pressure against d6. Another try coud have been Bf2, trying to exchange the nasty knight who prevents Rh-d1, to increase the pressure on d6... but I can't figure out the pan behind this move.

White, by a4 instead of Ba4, lost the possibily to influence the white diagonal with the lsb from b3. This would have been useful now. Even Bd5, hoping in NxB, could have worthed a try.

Wonderful. I see this move as a preparation of c3, though. Or Bf2.

a6 is a massive mistake, as it allows Nb6 check, and if Kc7 then Nd5 check. d5 is another mistake as it allows the Knight on a8 to escape via c7. I think b6 is the best move for black in this position. He has to round up the Na8 before white opens up the d file.

Black can consider d5, thinking that the knight can retake it and still be well placed, but... it's very tactical the situation after 19... d5, 20. RxNe6, dxN, 21. Rxd5+ (almost forced). Consider black is about to leave the white knight jump or lose a piece, if I am right, whatever he plays. // As for the text's, I don't really like it. I would have though about a6 instead, where white's lsb can retire or exchange, impoving my pawn's structure.

white takes the chance to enable his plan, but it still has to accept to restructure black's pawns after dxe. But black doesn't fall into the trap. After this, c3 would have been very strong for white.

I'd love to "restructure" my pawns by taking with the e pawn, but then my d4 pawn is pinned and he can just take my knight on e3.

I like this move!