I finally had some time to look at this myself; the following patch fixes the problem on my system (Debian unstable; wajig 4.0.3)
$ diff -uw /tmp/__init__.py /usr/lib/python3/dist-packages/wajig/__init__.py --- /tmp/__init__.py 2022-07-17 17:06:15.292801740 +0200 +++ /usr/lib/python3/dist-packages/wajig/__init__.py 2022-07-17 17:06:35.828934854 +0200 @@ -712,6 +712,7 @@ function = commands.purgeremoved parser_purgeremoved = subparsers.add_parser( "purgeremoved", + parents=[parser_teach], aliases=["purge-removed"], description=function.__doc__, ) Cheers, Holger