At the time of the release, people were wondering how the Pro Display had sufficient bandwidth to drive it. Big Sur came out at the same time.
Everyone using monitors with DSC 1.4 to the extent of their monitors capabilities saw those capabilities crippled and remain crippled ever since.
Those devices continued to work fine under Catalina.
Downgrading DSC from 1.4 to 1.2 on those monitors gave "better" performance (i.e. higher than without DSC, and higher than they were with DSC 1.4 with Big Sur+).
Hundreds or more bug reports were filed. This affected users of different Macs, different GPUs, different monitors.
The only common thread was DSC 1.4 and Big Sur (Ventura, Monterey... this still hasn't been 'fixed').
Given that the Pro Display and Big Sur came out 'together', it's very hard not to draw the conclusion that Apple found "sufficient bandwidth to drive the Pro Display XDR" by making changes to their "DSC 1.4" implementation that only the Pro Display understands.
FWIW, I ran `/S/L/E/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a` on Catalina and Ventura and diffed it.
MacOS Ventura seem to properly sense that monitor supports DSC: "DSC Support: 1, DSC Algorithm revision: 33" , but then simply disables it anyway: "DSC Enable: 0x0".
Interesting. And with maybe a little sense of shame, while I still have those monitors (27GN950-Bs), I actually have a Pro Display XDR on my M2 Studio (and the 2019 Mac Pro before it).
Torn between not wanting to support such behavior, and being one of the minority that could actually justify the Pro Display (photographer who works in at times challenging light environments).
When it came out people were wondering how they were driving it at 6K HDR10.
If you can dial your monitor back to DSC 1.2 things get slightly better.... I could do 4K 120 SDR, or 75 HDR.
Also applies to Mac Pro - my 2019 cheesegrater was affected, too.