Correct me if I'm wrong, but I thought some auth integrations don't work
with plack

I think you're talking about bug 17776 (Shibboleth auth broken in Plack). I bet people who set Shibboleth will be able to disable Plack if they need it on fresh installs.

Anyway, I take advantage of you mentioning it, and would like to say that we need better tools for testing such bugs.

Regards