For me this was a surprise -- I use [make that used] a 32 bit copy of 10 for a few older apps & testing, & when I couldn't get the AMD driverset installed, I went looking for a reason. It's there in their driver release notes, going back to June of this year I think -- from the notes for their latest drivers, released earlier this week:
"Note that AMD Radeon™ R9 Fury, Radeon™ Pro Duo, and Radeon™ RX 400 Series graphics are only supported by Radeon Software Crimson Edition on Windows 7/8.1/10 64-bit."
I picked up an AMD RX 470 for a good price, buying it because I *think* I had started to see the early stages of problems with my old 7870. The no 32 bit support had me seriously considering returning the card for a Nvidia 1060, but in the end I decided not to part with the extra $40 it would have likely cost -- I'll run an Insider build of 10 64 instead of win10 32 bit, & stick with my VMs for 32 bit Windows.
Nvidia has 32 bit drivers, & refers to them in the manuals or docs for the cards, so I assume there's no reason for dropping 32 bit support other than cost cutting on AMD's part. That they did drop 32 bit support will probably only effect relatively few people -- using Google I couldn't find reports of anyone else trying to use a new AMD or Nvidia card with 32 bit Windows. If they expand that trend to more of their GPUs is when/where lots more folks might be effected.
When swapping add-on graphics cards there's a handy utility to use, called DDU -- it removes a lot if not all leftovers so you start fresh. It can work with the drivers for graphics chips [GPUs] that are part of the CPU package, but IMHO you should be careful & research 1st in that situation, e.g. some Intel graphics drivers need to be uninstalled like an app via Programs in Control Panel. In my case I went the Control Panel route, which started the AMD installer, when it finished booting into Safe Mode to run DDU for any leftovers. Note that Msconfig still works in 10, but that the boot loader resets itself to boot normally when you next start Windows.
guru3d[.]com/files-details/display-driver-uninstaller-download.html
With add-on graphics cards cooling is often the name of the game -- it's often what differentiates one brand/model of card from another using the same AMD or Nvidia chipset. To stay quiet, many cards nowadays are set up so the fans don't turn on at all until the GPU reaches somewhere around 60 or more degrees C. One problem with that strategy is that you've got something inside the PC case that's already near the max temp you like to see for some Intel CPUs -- it's not the temp where they'll start throttling down, so they can get hotter, but a safety margin is a good thing. And as they say: "There's an App for that". Originally designed more for over-clocking, it includes the option to use a different fan profile.
guru3d[.]com/files-details/msi-afterburner-beta-download.html