Creating users by CSV upload shows the message "Auth plugin not supported here" when the auth column contains a plugin that is enabled but doesn't return true for the can_be_manually_set() method. However, the user is still created with the supplied authentication method.

To replicate:

• enable an auth plugin that doesn't support can_be_manually_set() (e.g. shibboleth, fc)
• use upload users to create a user with auth set to the auth plugin above
• confirm that the user is created and the confirmation shows the "Auth plugin not supported here" message
• edit the user's profile and confirm that their authentication method is set to the "unsupported" one you supplied

As to the expected behaviour, it's not clear to me what can_be_manually_set() is supposed to do, but it seems as if it should either not show the message or not create the user.