Atm I am somewhat unfit (relative to my past/potential) so when I do a round on the pads in muay thai/boxing which could be 2-4 minutes, my heart rate sits around 180-185 for the duration of the round. Often, sparring is actually more relaxed than pads because the coaches like to work us hard.
I have noticed that if my training meaningfully reaches 173+ I will usually feel like a vegetable for several hours after, usually the whole day. I get endorphins and heightened awareness around the 150-172 region.
"Max effort" is kind of meaningless because it means different things to different people - when I was 16 my swimming coach pushed me with sprints until I puked, not THAT uncommon amongst competitive athletes and I was a little out of shape that one time. I rarely see people push themselves that hard during HIIT classes, myself included...