It's not a niche accessibility need, it's a universal accessibility need that's been commonly understood for decades. Insufficient differentiation is one of the factors that originally drove the increase of colour count (and later, as display hardware allowed it, resolution) in icons 30-ish years ago.
This trend is not driven by universal preference for monochromatic icons but by the cargo cult that UX has become.
Also, Blender is not a grocery list app. While disregarding novice users does result in an application that's impossible to learn and that's obviously bad, it's equally unproductive to optimize an interface for people who see it for the first time. Blender is the kind of application that you spend tens, if not hundreds of hours learning before using it productively (let alone professionally). That's the target audience you're designing for, not people who download it and uninstall it if they get bored in the first thirty seconds.
Plenty of professional tools (Photoshop, Final Cut, Figma) have monochrome icons without it being a usability issue. These are all content creation tools, including Blender. The UI and content should not be fighting for attention. It's clear which of those two should primarily be on display.
I think it's entirely possible that Blender just has poor icons - but it seems demonstrably false that monochrome icons are inherently inaccessible.
My best guess is that Blenders real issue is a lack of structure and clear grouping. Providing the icons are in a logical place, it's easy enough to find the correct one, without them needing to be visually distinct on literally every dimension. But if there's no logic to the placement and the icon you're looking for could be one of thirty, then I agree that's an usability issue - just not that the icons are at fault.
Blender has always been very keyboard driven, but I don't use it often enough to have most of the shortcuts memorized. Even those seem to have changed, though. I'm sure there is a setting in the preferences to restore the old shortcuts, but it doesn't really feel like an appropriate thing to do with such a complex piece of software.
I know Blender has a "reputation" for being difficult to use. It used to be earned, over a decade ago, and it was indeed an impediment to getting new users. I felt like--as even just a casual user--Blender had hit a good sweet spot of organized, predictable, and powerful. It's just the very nature of 3D modelling to be complex, and it feels like most complaints about its interface these days are just from completely new people who don't understand this.
That seems to be one of the dangers of Free Software. Having no monetary barrier to entry, you get a LOT more first-time users. Without the sunk cost of having spent several thousand dollars on 3DS Max or similar, they don't have an emotional incentive to stick through the learning curve yet.
I've spent so much time watching videos and being confused because the fields they show are not in the same place in the version I'm using. It's one of the reasons I choose to stick to the default preferences as much as possible: I don't use Blender often enough to know exactly what I'm doing all the time, so I want my system to match what other people are presenting.
GIMP did this too and it absolutely blows. Thankfully you can revert back to the "legacy" icons.
For 2.8, all the Blender icons were replaced with monochromatic ones. This is a very popular trend and a lot of programs are replacing their icons in this way, so it's obviously fine for most people, and I realize this is probably a niche accessibility need I have.
But, to use the new icons, I find I have to check each one every time to find the one I want. Instant recognition is no longer possible. For me, this extra cognitive load makes it difficult to use Blender for more than a few minutes at a time.
Blender is a fantastic product and one of the best examples of what open-source can be, but I for one will be appreciating it from afar, and remaining on 2.7 until there is a way to get more usable icons.