This is a long complicated method. It is quite possible that in the future someone will add to it, likely at the end of the method. This early return could cause newly added logic to be skipped. To protect against that, I'd prefer you put your logic inside of an if and not early return:

if (!modalSubnode) {
 // your logic
 ....
}

This is cheaper than iterating over elements and should be done first.

We are looping over subviews and elements for each subview. It would be more efficient to put the elements in a set so lookup is O(1), or use a smart diff algorithm to get only the subviews not in elements and iterate over those.

[elements containsObject:subview] is likely the most expensive operation in this loop, so it should at the very least be moved to the last possible check. e.g.,

if ((subview.isAccessibilityElement || subview.accessibilityElementCount > 0)) && ! [elements containsObject:subview]) {
 [elements addObject:subview];
}