Mechanically impossible to solo
I think it would be keen if instances that can't be soloed without a hireling or party member were flagged more consistantly than they are currently. I have stumbled across several that I got stuck in because i couldn't be in two places at once, but did not state they required a party. "Requires at least one part member or a hireling" in the description would be helpull. Spending an hour clearing out a hardass instance just to get stuck because it's set to need an extra party member pretty much utterly sucks.
Your doom awaits, enter not through this portal
There is a difference between challenge and frustration
Frustration occurs when you put effort into something you can never accomplish
So if there are plates or something that would prevent soloing, then some indication should exist
That said, I think the warnings are too technical and do give away some of the challenge
So, when you get to the door and your level is too low, or your Int is too low, or you don't have a hireling, or 3 more people or the purple panda of doom, or whatever, simply state on the door:
Your doom awaits, enter not through this portal.
And err on the side of caution, even throw it on some very simple quests.
This way if you want to solo, you can just skip all doors so marked, ask on the advice channel why it says that, look it up, or even give it a shot. At least you were warned.
Then you won't be frustrated, but you can be challenged