Where can I find Yennifer or Triss once I have completed the main story? I've looked every where but cannot find them. Though they could be in the city of Novigrad but they weren't there.
I believe that since I romanced Triss, that they should at least have points in time to wear you run into them. I think that Yen is up North helping the Nilffgardians and Ciri says to you during a mission, that she wanted to be just like the traveling elves. She said that MAYBE after all of The Wild Hunt business was taken care of, that she'd run off with them and not give a lick about anything. To just travel for the fun of it. But in the cut scene where it tells you what all goes down before you meet Ciri to give her the special ordered sword, it says that Geralt and Triss moved to a disclosed area in isolation and Triss became the advisor in Kovir and they lived happily ever after. So, i'm not sure if CD Projekt Red removed the characters for a reason or just if they weren't sure if a glitch were to occur to where it would put you back into the main story and screw up your game.
Considering the story itself, I'm not sure about Yen whose theoretical location greatly depends on the ending but I believe that Triss departs to Kovir to become its king's advisor in any case, so it's logical she's nowhere to be found.
If Geralt romances Yen, they live together at an undisclosed location. If he doesn't and
Emhyr lives, she probably serves as his advisor and resides in Nilfgaard (remember that Philippa asked Geralt to take Yen with him because she wanted the position for herself).
Considering even as little as the above, the easiest way of dealing with those multiple outcomes while letting the player finish the secondary quests and contracts without adding new locations and conversations, is to let them simply disappear.
It is indeed unfortunate because one (or at least I personally) would expect that a couple of final conversations reflecting upon the whole adventure would be adequate and not too much considering the length of the whole story. I would appreciate at least a tiny location where I could meet either Triss or Yen, maybe Ciri
if she survived and became an almost-witcher,
and have one final conversation, much like in Mass Effect 2. Perhaps even do some contracts and then return for their witty remarks and some sexy time.
I got the ending to wear Ciri becomes the Witcher. I didn't know Triss/Yen/Ciri were to vanish from the side quests.
I romanced Triss and it said in the clip informing you of what happened before you meet Ciri (after the tower) that Geralt and Triss moved away into isolation and lived happily ever after.
Best thing to do is run a new game plus and create a save right at the first preparations phase in Novidgrad. Possibly, even at Kaer Morhen, when everyone is there. Then go off and do nothing, but side quests and the various expansions that come out. It's kind of like Metal Gear Solid V - The Phantom Pain. You can keep your favorite buddy, Quiet, but you can't finish the story. If you finish the story, she's gone. What a stupid design because the majority of your gaming hours goes into post-story play, so these NPC are only interacting with you for such a small percentage of your gaming hours, and they become a distant memory. I prefer the model that Mass Effect uses. Sure...further dialogue comes to an end, but you may repeat old conversations with them, perhaps, even call them to your cabin to hang out and/ or bed them.
Conclusion: I think CDProjekt was not sure as to whether or not they wanted their post-story state to indeed reflect post-story, or do what some games do and revert back to pre-ending state...therefore, they simply avoided the issue all together because other than the disappearance of your companions, which everyone assumes is a result to the post story outcome, nothing else about the world's state reflects post story. It's not like Skyrim where people are praising you for being the hero, or like Fable which does similarly and thereby giving you the impression the world reflected around you is indeed post-story with some previously accessed areas even being closed off, as a result to destruction that took place during the ending etc.