There are several deep canyons in the game, such as the one due east of Daytower Gate. In general if it looks like Aloy wouldn't be able to get back out, I assume the feature is just for decoration and don't try to go in.
In the case of the Daytower canyon, however, using Focus shows a destroyed machine (with the mostly trustworthy
Valuable Resources Detected tag) way out on the ice. So I went to investigate via a route to the bottom that did not involve any overly large drops, but Aloy died as soon as she touched the ice.
Is there some definitive way to know which areas in the game are off-limits? (Apparently it's not the presence/absence of interactive items!)