http://dosdude1.com/software.html
The guy that creates these patches is doing a huge service to the Mac community. I’ve successfully installed High Sierra, and now Catalina on my 2009 Mac Pro using his patches. There are a couple caveats - you have to update your firmware, you have to install a Metal graphics card (which often means you lose your boot-up screen - not a big deal to me), and SIP has to essentially remain disabled on your system.
It’s not a *super* simple process, but not terribly complicated either. And having upgraded the processors and video card (total ~$500) my machine just got about 40% faster, and like I said I think will keep working well for quite awhile still.