Yes it can run fine, but soon as you get a certain 'critical' update to scan your 'incompatible - too new' CPU they will recommend Windows 10 and halt windows updates.
Yes really any issue would be if there are drivers for your MB for windows 8 (which there should be) the cpu would be fine.
i have i5 8400 i used once windows 8.1 with it its says unsupported hardware so is there any chance to have latest windows 8.1 that support for coffee lake with also asrock z370 pro4 all drivers support for win 8.1 x64-bit ? cause i dont like windows 10