I quit early on a century yesterday, got 85 miles instead. It was pushing mid to upper 90s.
I had an early start and it was nice for a while but a wind slowed me down and I had one pass near my house at the end, so I just went back. No need to grind when I'd already had a full day.
For me it's a combo of speed, wind, and heat.
A 95+ day uphill or against the wind, moving slow, not fun and there's cold beer in the fridge.
95+ with a nice decent or tailwind on the way home from a good ride is just fine.
I'd never start a ride in those temps. Maybe 90, if it was cloudy.
One of the best race results I ever had was a 100+ day. I don't think I was particularly fast but everyone else started falling back. I suffer well, better than most, but I don't like to.