This is really scary. I am very sorry for the loss of the people.
The only slot canyon experience I have had was last October in Escalante. I checked with the rangers first RE the weather that day and the day before. After being told I was "probably" OK I drove to the canyon and went in. I must say that when I was in about 1/2 mile I ran into pooled water and turned back. I later ran into a young couple who navigated the entire slot. They said they waded through knee to thigh deep water. This did not seem too smart to me as I could not see the bottom of the pooled water I refused to enter!
Slots are fun and adventurous, but sure can be dangerous.