I never had the impression that the Mercator projection, or indeed any particular world map projection out of the dozens in common use, had any kind of official status that needed to be removed. It seems obvious to me that different map projections have different tradeoffs, and that any particular publisher of a world map can and does use whichever one makes sense for the information they're trying to convey.
This was also exactly what I was taught in school when I was 10 years old. We also saw examples of other maps from other parts in the world that put their location front and center.
That this map is a derivative of "a decolonization map", should be clear to anyone that this is just political posturing and not anything of actual use.
I assume the UN is a major publisher of maps, often featuring them on all sorts of pamphlets, websites, and if television is to be believed that backdrop for smoke filled meetings.
It makes sense they'd have a default and this would switch it.