Wasn't Internet Explorer out of support?

IE11 is still supported at Grade A & Grade C levels: https://www.mediawiki.org/wiki/Compatibility#General_information. Some new JS-only features don't need to support IE11, but for something as critical as the default skin on many wikis, basic layout should be working

In T309711#7977284, @ovasileva wrote:

Given the conversations around dropping support for IE11, I think we can drop the priority here to high

Note those conversations are around dropping IE11 from Grade A to Grade C[0] (i.e. no JS) whereas the issue described here in a CSS reading issue. I don't know if our guidance on UBN's in clear enough to say if this is one or not, but either way this is a very awkward regression for a not-yet-insignificant percentage of our readers, especially in specific countries (i.e. this should definitely be blocker to enable on any Japanese wiki).

0. "Starting in March 2021, new JavaScript features on any Wikimedia project will not be supported on IE11, as documented at Compatibility/IE11. This is the beginning of downgrading this browser to Basic (Grade C) support." - https://www.mediawiki.org/wiki/Compatibility#Special_treatment_for_IE11

This will be fixed by T303484 which will make IE11 appear like a mobile device (header, sidebar (collapsible), content, footer stacked on top of one another).