My $0.02 as a pull up specialist has-been
I used to be exclusively thumbless/thumb over up until several years ago. My crowning achievement was 20 pull ups with an additional 50 lbs. That was around 15 years ago. I've done reps with 135 before the 'Beast' ever existed. I've done 200 reps in a day GTG style for a few weeks just to see if I could. Nowadays, the pull up has been a standard auxiliary in most workouts rather than a primary movement. All of that past training was done 90%+ thumbless, but if I could go back I would have done it mostly with a full grip, thumb under and around the bar.
Why? I learned late in my pull up shenanigans that I had much less elbow and shoulder pain when I actually used my thumb and crushed the bar that I hung from. I liken it to the difference between dead lifting with and without the thumbs, however in the case of pull ups you are merely hanging from the bar when you don't use your thumbs; you can only squeeze it so much. When you grip and crush it, you and the bar become one. I can actually tell a big difference in the mind-muscle connection - I can feel my lats working harder and I can actively depress my scapula better under heavy loads. I also attribute the reduced elbow pain to the fact that crushing the bar stabilizes the wrist. Once you lose wrist stability - often from swinging or not pulling in a smooth trajectory - compensations occur at the elbow. This is more likely (though not guaranteed) to happen with the thumbless grip.
At the end of the day, one will do what works best for them. I used a thumbless pull up grip for the better part of two decades and have dropped it after learning how to make the full grip work better for me.
PS: I also switched my barbell military press grip from thumbless to full around the same time for a lot of the same reasons.