That was a concern when I had an ultrawide monitor. Fortunately I remote into work and the screen share happens on that remote side. So whenever I had to screen share, I would take Citrix Viewer out of full screen and size the window to my best estimate of 16:9. I don't think anyone ever knew that I was using an ultrawide monitor, though I do wonder if they ever noticed that the aspect ratio of my desktop shifted a bit between each sharing session.