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.