Not quite; as Fili Wiesel (original poster in this thread) documents in the doc he wrote and quoted, the core of webapp2 works fine with Python 3.7 -- it's just several "extras" for app engine, such as integration with memcache or the users service, that are not compatible. At
https://github.com/GoogleCloudPlatform/webapp2/issues/137 you'll see that Fili is looking for helpers to improve the situation (in the upstream, open-source repository), and Thea Flowers, a Pythonista and Googler in developer relations for Google Cloud Platform, has signed up as such a helper.
So, one might sign up to help out, in the spirit of open source.
Alternatively, of course, one can pick some other lightweight framework (Flask is, I believe, by far the most popular) and give up on the webapp/webapp2 tradition. If writing an App Engine app from scratch, that might be simplest; an improved webapp2, however, might make it easier to port existing webapp2 projects from Python 2.7 to 3.7 (though other issues such as ndb may loom larger; for that, maybe
https://github.com/Bogdanp/anom-py might be of some help).
Alex