this info has been all i needed so far.
https://www.anandtech.com/show/11182/how-to-get-ryzen-working-on-windows-7-x64
i've put win 7 on both new ryzen and intel builds so long as i followed the usb2 advice.
of course this will begin to be more of a problem as drivers stop being made for win 7. but if you do your homework you can ensure driver support. you may have to do some serious detective work but normally you can find compatible drivers. may have to pick a different mobo to ensure you can get drivers.
for instance, msi may not put out a win 7 driver for the audio chip on the shiny new board. however, often the chip has win 7 drivers from the manufacturer page or possibly an older mobo has the chip on it and has win 7 drivers. can take time and may not get 100% compatibility but i've not had problems getting what i needed this close to support ending.
had to do the same thing when vista came out and people wanted xp back. same when win 8 came out and people wanted win 7 still and so on and so on.