Faolyn
(she/her)
Exactly.5e never explicitly grants a class a proficiency and then prohibits its use.
Wrong. A druid won't use metal armor, but is proficient in all Light and Medium armor, regardless of what materials they have.In the case of Druid, the class lacks metal proficiency.
Name one race or class that only grants part of a proficiency.There are many examples of races and classes using a proficiency (including skill proficiencies) in a special way, under specific circumstances.