Generally I’d say a squishy class (or rather, a build, a class isn’t inherently squishy) is a class that either gets bursted down easily, be it through lack of damage mitigation or low health, or a class that struggles to recover health once lost.
The issue with the first type of squishiness is that it’s hard to judge until you actually play it in difficult content. You can be running around on Ultimate murdering absolutely everything without your health even moving an inch, to the point where you may wonder if enemies are even hitting you, and then you run into a tough encounter and go splat.
Say a Belgothian Blademaster does have a decent number of defensive layers thanks to high dodge, high defensive ability, full armor absorption, % damage reduction, high crowd control resistance, and the ability to burst out a ton of damage to recover health, but it only takes a few unlucky hits in a row and you can be dead, because a lot of your defensive mechanics are all or nothing (DA, dodge – either you don’t get hit, or you take full damage). Alternatively, you can be very weak to a certain mechanic…an enemy boss simply spawns with a nice collection of retaliation gear and you kill yourself on them within the blink of an eye because you attack insanely fast and with multiple damage sources at once, each of which will apply full retaliation damage to you instantly.
An example of the latter type of squishiness (lack of recovery) would be something like a grenadier build. I’m currently playing a full grenadier/ulzuin’s chosen purifier (Stun Jacks, Grenado, Canister Bomb) and you can’t lifesteal directly off of any of those attacks. You take damage, it’s going to stick. Where other builds can have their health jump up and down like crazy in intense encounters, here you have to kite and pick your moments to shit out damage cause if you’re reckless, you’re splattered on the walls.
Once you get over the hurdle of being one shot by dangerous enemies (doesn’t happen as much early game, there you struggle more with sustain) you can still end up being squishy if you can’t recover the health you lose.