One example I can think of is that Direfrost Keep had its loophole closed, so you'll need another player to stand on the pressure plate.
All of the non-DLC dungeons are still soloable as are some of the DLCs.
Hello, I play a Stamplar and really enjoy playing solo. Nothing against anyone, I'm just a loner. I have read some solo guides and builds and each of them say that these builds are tailored to solo dungeons and trials and whatnot. My question is, what content can I definitely NOT solo even if the stars aligned?
You are correct. The post just above theirs points out a non-DLC dungeon that is probably not solvable, though I have not tried. Direfrost requires two players to stand on separate pressure plates at the same time.
Edit: Adding to this, FG II has a boss mechanics where one person gets pinned and will die (maybe almost die if at full health) if not released. That is likely a game stopper for trying to solo that dungeon.