If you have a slightly older nVidia GPU (1000 series or older), it will work in High Sierra via nVidia's in-house drivers. But you will never be able to upgrade past High Sierra.
Edit: honestly I have no clue why I am downvoted: I really would appreciate an answer here.
AFAICT the answer is that Nvidia and Apple are not the best of friends, and Apple decided that they would not sign any newer drivers.
This may have been at least partly because it restricts the numbers of people who try making hackintoshes out of existing machines.
I think this is probably a factor in the lack of support for intel Wifi too - it keeps a lot of laptops from neing able to 'just work' as hackintoshes too
People think this based on a vague statement from nVidia's PR. It doesn't make any sense to me. nVidia has continued to release (minor) updates for the High Sierra drivers, and Apple has been perfectly happy to sign those. Also, this is a technically-advanced audience that would probably have no trouble installing unsigned drivers.
I really think that nVidia realized they would have had to do a major driver rewrite for Mojave (because much more rendering goes through Metal) and decided it wasn't worth the effort. The Mac drivers were clearly a very low priority for nVidia; it took the better part of a year before the company added compatibility with their new Pascal graphics cards, and even then the drivers had all sorts of bugs with applications like Little Snitch[1]. It's even possible that nVidia had some knowledge of the upcoming ARM transition, and decided to cut their losses ahead of time.
1: https://www.obdev.at/support/littlesnitch/483957225401957
csrutil disable && csrutil enable --without kext
Now unsigned kexts will be allowed to load (while leaving the rest of System Integrity Protection intact).For most software this requirement would be a massive problem, but I'm not sure that applies to the type of person installing 3rd party graphics cards in Macs...
Oh well, it is what it is...
https://9to5mac.com/2018/05/05/nvidia-egpu-thunderbolt-macos...
Not sure if the same hack could be used on a Hackintosh (with an old enough card). But for now I would just use High Sierra if you have an Nvidia card, unless you really needed something in a later OS and the hack made it possible. I downgraded that 2012 to High Sierra, and keep other older machines on it as well (didn't have a choice with my 2019 16"). It runs very smoothly and currently has enough software support (e.g. latest Office 365 still supports it).
For now, there are no Nvidia web drivers posted for anything after macOS 10.13.