> In the case of Claws Mail an argument could be made for upgrading to a never version of GTK and get a facelift in the process. That could also hurt the users, if the UI move around to much, or break existing work-flows.

Since last year, they have a release branch (4.x.x) built with GTK+3. I know GTK 4 is on the way, but I think nobody is using it nowadays.