Oh, and you can do that even in the official application? I thought it was impossible to be logged into multiple Teams accounts at the same time! (That was one of the reasons I was using Teas in a browser, being able to open another instance in a private window.) Or did they finally fix that, at least?

Access Teams? Do you need anything more than opening the Teams web app in a browser?

uMatrix browser extension. It has been marked archived by Gorhill, last release is two years old, you are supposed to just use uBlock [Origin]. However, it still (luckily) works fine and is exactly what I want. (Sure, I won’t install this for my parents.) The GUI to simply choose what you want the site to be allowed to do is perfect.