Werkzeug just released version 2.1.0 on March 28, 2022 (4-5 hours ago as of writing this). When using runserver_plus with that version of werkzeug, the development server cannot be started.

As part of werkzeug 2.1.0, some previously deprecated code was removed, including werkzeug.server.shutdown
From the werkzeug release notes: "Remove the non-standard shutdown function from the WSGI environ when running the development server. See the docs for alternatives."

In django-extensions runserver_plus.py - line 326 we have this:
del environ['werkzeug.server.shutdown']
which results in a KeyError since shutdown was removed

Rolling back to werkzeug <2.1 (2.0.3) resolves the issue.

This is the first time I've looked into code for runserver_plus -- but would be happy to provide a PR if anyone with more familiarity can provide some rough guidance.

Also, while making this change, perhaps the preceding line is from runserver_plus (if not options['keep_meta_shutdown_func']:) is also no longer needed as well...?