Note to those not liking this, there is an option in the preferences to revert to the old Vector skin. No idea how long this will be but setting that preference may give them feedback that some users prefer the old layout.
To what extent Wikipedia's UI/UX assessment team are aware of this and can note a strong defection from any new features even where that defection represents only a small fraction of visitors I don't know.
Worse: sufficiently annoying / offputting features can simply lead to defection from a site entirely. A major problem with A/B testing is that if the cumulative effect of features has been to drive away those preferring the path not taken, then all your A/B tests are doing is validating survivorship bias.
(I'd give my experience with Reddit over the years, which has lead me to all but entirely avoid the site, or more recently a rather drastic turn in Twitter's dynamics affecting many long-term participants, as cases in point.)
Of the specific changes Wikipedia are putting into place, the maximum line length is a style change I've applied myself for about a decade now using various CSS browser extensions. I'm not much a fan of pinned headers, though that would be less an issue on desktop than my e-ink tablet. For that, I prefer the Desktop layout generally, but zoom the screen to cut off the left-hand sidebar entirely, which works fairly well until such time as I want to view an image fully zoomed.
I do find the sidebar useful on desktop. The present Mobile layout is annoying in multiple regards though it might be useful on a smaller device (I'm using a 13" e-ink tablet, which is generally not constrained for space.)