Apple's policy is to not support 5+ year old hardware even if it would cost them virtually nothing to do so. Sometimes specific hardware is justifiably required for new software (Quartz Extreme, Airdrop, Metal), but mostly it's just Apple literally restricting a machine-identifier from the MacOS installer in a plist for no good reason other than their policy. They did indeed let the MacPro5,1 slide because they were so late on the new one, but it doesn't really justify the literal planned obsolescence policy of artificially excluding something on a plist for absolutely no technical reason whatsoever.
I'm gonna get a MacPro7,1 -- mainly because I don't have time to mess with installer hacking for my MacPro5,1 every time I have to update the OS.
But if you think about it, this thing is going to cost $7,000 and this is basically what the change is compared to my current machine:
CPU 2.8x4 -> 3.5x8
RAM 24 -> 32 GB
GPU RX 580 8GB -> 580X 8GB
HDD 1TB SSD -> 1 TB SSD
Now yes, it's a total architecture change and it's going to be unbelievably faster. BUT -- if my current machine worked with Catalina... It would be "Good Enough".