It's a combination of hardware and software support.  Because a lot of functions of the motherboard are still supported by different operating systems, certain parts of the motherboard may work perfectly fine with Win7, Vista, and sometimes XP.  However, some hardware is designed with functions that have limited support in different operating systems, which could be fatal to support on an older OS.  At some point, the technology just requires certain software conditions to work.
 
Because of that, you may find that a product will only work with Windows 10, because Windows 7 (or other) may not have certain processes or environments needed by either hardware or software to function properly.  Some of that is determined by the component manufacturer, like Intel, and some is determined by the manufacturer. 
 
A third reason is that component and hardware manufacturers do not want to use older operating systems that may have compromised security status, because the OS is no longer supported for security updates. 
 
This of course, just refers to literal compatibility.  The discussion about how effective a motherboard might work in an unsupported operating system is an entirely different conversation....
 
Linux is a different matter.  So long as you can locate all the proper drivers for the motherboard, you should be fine, but EVGA cannot provide troubleshooting support for it.

WaitingInLine
I honestly wanted to avoid win10. But if there could be some catastrophic issues maybe I should secure one just in case. I'm annoyed with windows enough, but it's better than nothing at all. 

Z490 Dark can run Win7, But need to have a basic idea of Slipstreaming updates, DISM tools (in this area you need to understand more, like boot.wim, install.wim and integration of drivers etc with a Windows ISO) that's basic knowledge on the matter. Esp with Win7 you have to know more, because the MSDN ISO is Service Pack 1, but after that there were so many updates to Win7 even the latest 2021 April patches and there were some mandatory patches like - SHA256 patches which enable your Win7 capable to use Microsoft Updates and NVMe generic driver patches (which work great with Samsung), then you need to have xHCI USB3.0 driver stack modified from Win8.1 from Win-Raid forums, to make the OS detect and use USB3.0 ports. That is basic, next you need the WinPE bootable and install Win7 through there. (Z390 Dark also can run and there's even a guide here)  however I will mention the easier one - Simplix pack integration with Win7 ISO this will get you all of the above but you need WinPE bootable probably.
 
Z590 ? I don't know, I'm waiting to see if anyone installs Win7 onto it. X570 also same but some changes to the process. Remember this is the last generation of HW that can run Win7, because next gen will remove the CSM options from the BIOS probably and EOL the BIOS entirely this has been the plan for Intel and MS since a long time, and with Alder Lake needing some sort of Scheduler support from Windows OS since the OS needs heterogeneous cores and not that hybrid drama. If you are curious on this visit Win-raid and learn more, if you are successful post it here !!
 
Finally I think EVGA BIOS and ASUS BIOS are great so no need of OS software to change a lot, everything through BIOS is best and you eliminate that Software from the equation, if people want to tinker with Processors, they should use Throttlestop over XTU. Also I cannot share links here, maybe my profile is too new so sorry, I couldn't share direct links for you, about the BIOS EOL you can search it "BIOS EOL Phoronix".