This bug was introduced in da9e4e6e54.
validate validate_plan_for_authentication_methods is already called
inside validate_authentication_methods_dict_from_api, conditionally on
settings.BILLING_ENABLED. This additional, redundant call runs
regardless of BILLING_ENABLED, and thus prevents a self-hosted server
from enabling certain backends in the organization settings UI.
The impact of this is limited - in order to encounter this bug, a
self-hosted server would have to first disable the backend in the UI, as
self-hosted realms are created with all backend flags enabled. A backend
doesn't show up in the org settings UI until it is first enabled in
AUTHENTICATION_BACKENDS in settings.py - that's why this is a rare
state. A sequence of steps like this has to be followed to reproduce:
1. Add the backend to AUTHENTICATION_BACKENDS in settings.py.
2. Disable the backend in the org settings UI.
3. Now try to re-enable it, which fails due to the bug.
114f13e0ee