You can still download "WAB, Developer Edition" (it's tucked away now but still available) and migrate projects from the formerly built-in version. We did this when the upgrade to 10.8 broke all our apps and Esri could not help us fix them. With WABDE, the apps generated are stand-alone, they install in any web server. Generally, you can treat them like a bunch of HTML files. Configs are just JSON files, so you can tweak them with a text editor. Of course they still rely on whatever ArcGIS services you put in the map.
I suppose in an upcoming "upgrade" that the "classic" map editor will go away and that will be the end of it forever. I've looked at the map files and decided it was not worth the work to modify them in a text editor.
Because we've been running WABDE since 10.8 we did not even notice WAB is not in 11.x!
If there were a Public Notification widget for EXB we would have migrated a year ago. But here we are.
I am retiring before WABDE !!! NOT MY PROBLEM !!! 🙂