I noted yesterday that the option to select an area of a board as the board thumbnail has disappeared in the Edge browser.
I reported this to the support team and will update this post as I hear more. While it is possible the support team will reply that this was never an option in Edge, I am fairly certain that I have does this in Edge before.
Edge version while testing: 121.0.2277.128 (Official build) (64-bit). I tested as both a board owner and team member.