I thought this would be worth knowing if you use Linux and compile yourself. This may also affect KiCad packaged for some distributions in the future.
Possibility to build KiCad with GTK3 UI library was added recently. Until that it was possible to use only GTK2. Because the legacy toolset doesn’t work with GTK3 (and isn’t worth porting to it) it has now been removed from GTK3 build. If you compile yourself and want to use GTK3 you’re probably not affected because you have been using only the modern toolset for a long time now.
But in the future it’s possible that some distros use GTK3 for KiCad (it will be v5.1 then). In that case those who have used the legacy toolset have to move to the modern toolset.