A work-around for screen and application sharing (at least for me) is to use the browser version of Teams (https://teams.microsoft.com/) with Google Chrome browser (version 87.0.4280.141 .)
I'm running Kubuntu-18.04.5 LTS, with a KDE desktop environment, on a Dell laptop with external monitors attached - but using the laptop's camera, mic and speakers. An earlier release of Teams for Linux (from many months ago) allowed screen sharing, but the current version that I'm running (1.3.00.30857, amd64) just brings up an empty selection area at the bottom of the Teams screen - where the sharing choices used to appear.)
If I instead use Teams in the web browser, I can select desktops or application windows to share, and it seems to work just fine (although I haven't tried letting others control my desktop.)
It is sad that the actual "Teams for Linux" application doesn't fully function, while the browser-based version does, but one of the advantages of that is any security issues are more likely to be fixed quickly in a browser than an apparently unloved Linux application.