Armor Proficiency. Anyone can put on a suit of armor or strap a shield to an arm. Only those proficient in the armor's use know how to wear it effectively, however. Your class gives you proficiency with certain types of armor. If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can't cast spells.
Currently the character sheet does not enforce this rule.
How to replicate:
Any caster dons armor without a proper proficiency (in my case, Warlock dons a shield).
Expected outcome:
Spellcasting should be unavailable or blocked, with a warning message that states that you can't cast spells because you are wearing armor without a relevant proficiency (ideally showing the piece of armor that blocks the spellcasting ability).
Actual outcome:
Spells can be cast normally, a spell slot is spent.
From PHB144:
Currently the character sheet does not enforce this rule.
How to replicate:
Any caster dons armor without a proper proficiency (in my case, Warlock dons a shield).
Expected outcome:
Spellcasting should be unavailable or blocked, with a warning message that states that you can't cast spells because you are wearing armor without a relevant proficiency (ideally showing the piece of armor that blocks the spellcasting ability).
Actual outcome:
Spells can be cast normally, a spell slot is spent.
Is your warlock a hexblade? They gain proficiency with shields.
Nope. Also confirmed that this can be reproduced with other armors without relevant proficiency as well, like donning a medium armor.