PHB describes the downsides of wearing an armor you are not proficient with:
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.
However, a shield is not armor.
The Spellcasting chapter says about armor explicitly, and not shields:
Casting in Armor
Because of the mental focus and precise gestures required for spellcasting, you must be proficient with the armor you are wearing to cast a spell. You are otherwise too distracted and physically hampered by your armor for spellcasting.
It seems a Sorcerer can easily wield a shield. What exactly are the downsides of not being proficient with shields?