The boots "The speedy Lightfeet" are considered as medium armor and warns for non-proficiency when equipped but have no in game effect.
[Linked Image from i.imgur.com]

you can still cast the spells and sneak attack while wearing them if not proficient.
[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]

while wearing actual medium chest piece armor spells are not able to be cast but sneak attack i still possible even if it gives you disadvantage on the attack.
[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]
[Linked Image from i.imgur.com]

the same logic applies to helmets, gauntlets and boots that are tagged as light/medium/heavy armor but does not impede the unarmored perk for the barbarian.

Last edited by TheSlaughterfish; 18/02/22 10:33 PM.