The only true full depletion bonk I can recall experiencing occurred during NCO academy at Camp Pendleton in the mid 1970s.
My aerobic base conditioning wasn't really suited for the terrain where we trained, the hills of Pendleton. I had mostly commuted by bike 20 miles a day, played racquetball, and trained for boxing in the gym a few days a week. In the gym my only leg work was skipping rope and moving around the ring for shadow boxing and around the heavy bag. I hated running and avoided it other than the relatively easy annual physical fitness test. (Boxing fans might be surprised how little roadwork some boxers do. Most boxing conditioning happens in the gym. Running is seldom really relevant, even for a boxer who "runs" as a defensive tactic. Unless you practice running backward, which Muhammad Ali did, normal running doesn't really work the same muscles. Skipping rope, lunges and squats, shuffling around a lot, with lots of bending from the waist, is more relevant to boxing.) Even without running the rest of the year I could pass the 3-mile PFT run and always exceeded the minimums for sit ups and pull ups since I did those in the gym anyway.
But I was definitely not in shape for the runs we did up hills/mini-mountains in NCO school. Totally different conditioning. After struggling the first hill run I started running the same hills every evening after classes -- in retrospect I should have done those evening hill runs only every other day for the first week or so. It was going okay but one day I changed my diet ("carb loading", rather than my usual near-keto diet) and didn't get enough sleep (I might have had watch that night, I don't remember). Bad combination of conditions.
I felt out halfway through the run. Literally stumbled and fell and could not get up. Not "didn't want to get up" or "I'll just rest here a minute." I *could not* get up.
As a kid on that poor diet I mentioned before I'd had lots of low blood sugar shakes, headaches, etc. So I knew how that felt. And between lots of endurance bike rides and three 3-minute nonstop amateur boxing slugfests, I knew what physical exhaustion felt like. But in those cases I could always keep moving, even if just barely. But that collapse during a hill run at Pendleton was my first, and so far only, true depletion bonk.
That was pretty awful. It felt like one of those waking-dreams where we're half-asleep/half-awake and feel paralyzed.
I've tried to avoid that combination of conditions: ill-advised dietary changes; inadequate rest; over-training on new physical challenges; pushing myself beyond sensible limits.
I've had a few long bike rides, especially in hot weather, where I could tell I would bonk if I continued. So I'd just stop, let the group go if it was a group ride, rest, eat, drink and find my own way home at a comfortable pace. I experience those very seldom now, especially learning what my sustainable pace is in my 60s.
And I've done a few fasting rides close to home just to get a sense of how I'd respond. It went well enough, no problems. Now I do that mostly on the indoor trainer -- easy zone 1 spins for about an hour.