If a jstest's @tags array includes a feature flag that is listed in fully_disabled_feature_flags.yml, that test will never actually run:

• It won't run on the "all feature flags" variants, because the flag is forced off there.
• It won't run on the "no feature flags" variant either, since the flag defaults to false.

So the test is dead on arrival: it gives the false impression of coverage while never actually executing anywhere.

We should add a check, similar in spirit to the existing feature_flag_tags_check.py, that flags any newly added jstest whose featureFlagXXX tag matches an entry in fully_disabled_feature_flags.yml, and wire it into the commit queue so such patches fail instead of silently merging.