Various versions of GTK which introduced breaks but didn't really change the core philosophy of how you build a GTK app. IMO, this is the preferable way to evolve an ecosystem.

The problem microsoft has is instead of making "Win32, but with these extentions or these APIs removed", heck even as a separate "framework". What they did instead was "You know what's hot right now? XML. So let's make an XML based UI framework. Actually, it's javascript and css, so let's do that. Actually, people really like electron so let's do that."

That is to say, it is possible and I dare say easy to migrate an application from GTK 3 to GTK 4. It's basically impossible to migrate a WPF app to UWP. You have to rewrite the whole thing.