@JeremyECrawford Sentinel Shield says, "While holding this shield", you get it's benefits. Does that mean wielded or just "holding"?
— deadlye (@deadlywuon) April 17, 2017
"Holding" means holding. #DnD #English https://t.co/HnnO2xtAi1
— Jeremy Crawford (@JeremyECrawford) April 17, 2017
Of course holding means “in one’s hand or hands”, so you might as well be wielding it.
“holding” and not “wielding” means wizards and warlocks could gain the magical benefits of the Sentinel Shield without incurring the penalty for not being proficient.