There have been quite a few threads on this forum related to Zoom.us screen sharing, especially flickering screens for meeting participants.
Well, I had the same issue and was going up and down the stack to find a solution. To the extend that I last week installed Windows in a separate partition for my Zoom sessions. 
And, as usual, the moment you throw something away, you need it, or in my case right after Windows was installed I found my solution.
The screen flickering is caused by the tearing prevention setting of the KDE compositor. Switch it off and the flicker is gone!
Given that this fixed a big pain point in an otherwise great product, I thought I'd share my solution.
Find the compositor in systems settings or type compositor in Krunner and set vsync accordingly.
