Whenever a shield comes up, your DM spidey senses should come up. The character shield does a relatively poor job handling the AC bonuses of a shield. On top of that, the rules don't do a good job explaining what requires two hands or how that can be abused.

For practical purposes... spell casting requires two open hands, using a two-handed weapon (or versatile weapon) requires two hands, and using a ranged weapon weapon requires two hands (this includes the hand crossbow, and there was an errata to include the sling).

Anything that requires ammunition requires a free hand to load (and reload) the weapon.

https://rpg.stackexchange.com/questions/65363/can-i-load-a-hand-crossbow-while-holding-a-shield https://www.reddit.com/r/dndnext/comments/3kqum0/sling_with_shield/

Spellcasting and Shields...

https://www.quora.com/Can-Druids-use-wooden-shields-and-still-cast-spells-in-D-D-5E https://www.quora.com/In-D-D-5e-can-you-cast-a-spell-while-holding-a-shield-or-wielding-a-two-handed-weapon https://www.reddit.com/r/DungeonsAndDragons/comments/7952nb/5e_tf_a_druid_holding_a_shield_and_a_quaterstaff/