is an RX 580 8gb GPU only compatible with windows 10?

No it has the following drivers available

Windows 7 - 32-Bit Edition

Windows 7 - 64-Bit Edition

Windows 10 - 64-Bit Edition

Linux x86_64

Ubuntu x86 64-Bit

I installed an 8gb RX 580 on my brothers Windows 7 64-bit system with a Core i7-3770k a couple months ago and I had no problem finding a driver or anything like that. In fact, back in the spring I had an RX 480 installed in his system and I had no trouble installing that either.

There was an issue a month back with the driver because it flat out quit working. Safe mode attempts, startup repair didn't fix it. There was no restore point either. A program called Display Driver Uninstaller was used in Safe Mode to remove the old driver, then a new driver was installed. This worked and the card is running again.

He had a game running, then walked away from the PC to go to work, then my other brother terminated the game and the PC had issues with the driver after that. So I don't think it was any sort of compatibility problem with Windows 7. Whenever an AMD card has been installed in his system, there has always been an issue with the driver.

when it says "minimum" it means MINIMUM. the drivers are NOT guaranteed to work with Win 7 and you'll get no support from the gpu maker for related problems

That would be uber ghey if it was