How much reduction in hydration requirement do you experience in cooler temperatures, let's say the difference between 75 degrees and 50 degrees at the same relatively hard effort? Generally, one 24 oz bottle/hour seems about right in moderate temperatures, but as we swing into a California winter I suspect the need drops to 1/2 of that figure.

Just wondering what others have experienced

For summer temps, on a typical hard solo 50 mi ride, I'll down the 2 full large bottles that I carry.
Cold weather temps, I still drink but not as much so, I'll down 2 regular sized bottles by the time I end a 40 - 50 mi ride. Gotta drink, even if it's cold b/c I'm still working and sweating. Really cold temps (e.g, below freezing) even if it's just a 25 mi easy spin, I still carry at least 1 regular size bottle and drink all of that by ride's end.

I always carry two full 24 oz bottles. On a 35 mile ride in hot weather I'll run out and stop for more and in cold weather I may not finish one. I let thirst be my guide. It is, after all, the sense that has evolved over millions of years to guide us as to when we need to drink. In warm weather the rate of water loss is high so you do need to mindful of it and drink at the first indication.

Another 2 bottles on all rides. Better to have than not have.

Your question leaves too many variables for a precise answer.
75F isn't so hot that you would experience extreme sweating - unless the exertion is extreme. So therefore, I would guess that comsumption should not drop much at 50F. Going from 85+F to 50 might change consumption requirements considerably given similar exertion.

Around here we don't consider 50F to be cold. Many/most of my summer rides are done around that temperature.

Back on track: I don't alter my water consumption much as the temperatures alter between 85F and 35F. My limiting factor is generally my ability to keep swallowing and get the fluid absorbed, not how hydrated I am. The only difference is how dehydrated I get per hour.

When I'm riding in a place where it is warm, like up to 110F, I make a point of drinking more, but I know at some point I will dry out unless I really force the issue by cutting back my output in order to allow some blood flow to the gut.

Amen brother! I've been severely dehydrated on a ride before and don't want that to happen again.

Just because it's cooler and you don't feel the sweat, you could still be losing significant fluid amounts from exhaling alone.

Weigh yourself before and after a ride. If you've lost more than 2 pounds, you probably are low on fluids. A rule of thumb is a pint = a pound.

FYI: When I was in the Army, stationed in Alaska, dehydration was a big problem during winter field exercises. Since they weren't sweating, and weren't thirsty they didn't drink.

Yes, I think half as much is about right for me, too. I've been carrying only one bottle instead of two the last few weeks. Of course, I've been cycling less, too, because of the cold and the shorter days.

I carry and drink as much water in cold dry air as I carry in warm damp weather. Cold and dry air creates extra loss of hydration from respiration, even if perspiration might be slightly reduced.

Winter weather can provide a greater need for preparation, including fluids.

If I lived in a dry and arid climate, I might reduce the need for water in cooler weather.

In cold weather, meaning below freezing, need to drink just as much water. Easy to get dehydrated in cold weather.