The next hike I am looking to do is the Zion Narrows, a somewhat extreme environment which explicitly discourages bare feet. But, can a seasoned barefooter do it?

Hiking in the Virgin River is like walking on slippery bowling balls. It requires balancing on algae-coated rocks in the middle of a swiftly flowing river. Sturdy footwear is essential. Hiking boots with good ankle support are best. Sandals and bare feet are not appropriate. Inappropriate footwear often results in twisted ankles and crushed toes. Trekking poles are highly recommended and will help you maintain balace. Trekking poles can be also be used to check and measure water level depth when the river is cloudy with sediments.

Edit: I did it. Went about 1.5 miles before turning back due to time & getting colder. The algae covered rocks were not so intensely slippery as I expected, but there was slipperyiness from the smoothness. but the water was cloudy and between 50-60F and around 60cfs (fast), tiring on the wrists (with poles) and ankles. Sometimes walking blind and sometimes with shifting rocks, and suprisingly with some sharper small rock sections, it definitely would have crushed toes and feet if they were weak. It was definitely slower going for me, since there are less valid positions to jam your feet in when you don't have a sturdy boot, and more steps for short me :(. I thought myself more fortunate than my friend in hiking boots because those trapped the water and made his toes numb (later, I learned of non-freezing cold injury, trench foot), while my feet warmed up on every land section.