Unless it's solely for balancing issues, I'm wondering why the shield proficiency is locked to the fighter, cleric and ranger classes (not even druid?).
The druid does have shield proficiency in BG3 and 5e. Were you trying to equip a metal shield though?
For example, I would expect a shield dwarf to be able to use a shield.
Shield dwarves, which is another name for mountain dwarves, do not get shield proficiency in 5e: