GNOME releases version 44

This is also a common issue with cheap and nasty docking stations, which only support DP 1.1.

In theory, if you had a bad cable, you might have DP 1.2 (hence YCbCr 4:2:0) or DP 1.4 available (hence DSC), but still be limited to 8.64 Gbit/s (HBR, 4 lanes or HBR2, 2 lanes), rather than the 17.28 Gbit/s (HBR2, 4 lanes) that DP 1.2 permits, but that's not hugely likely; you're more likely to have a bad cable that can only do 5.184 Gbit/s (RBR, 4 lanes), at which point you're limited to 8 bits per pixel at 4k60, and thus even YCbCr 4:2:0 (12 bits per pixel) won't work - only DSC will, and even then only if both ends can do DisplayPort 1.4. If you're really unlucky, you only have 1 working lane, and can do 4.32 Gbit/s (HBR2, 1 lane), and need both 4k 30 and YCbCr 4:2:0 to get a 4k picture (because you only have 7.27 bits/pixel at 4k60).

In contrast, 4k30 is possible even on the really bad cable using either YCbCr 4:2:2 or DSC. It's also possible on a good cable doing DP1.1 limits because a device in the chain is DP 1.1 only (by design or configuration). Thus, falling back to 4k30 is a good choice if your setup doesn't do 4k60 RGB 8:8:8 - the only time you could do 4k60 with fewer bits per pixel is if you've lost 1 or 2 lanes in the cable, or if you've got a cable that's good enough to handle HBR but not HBR2 (which is exceptionally rare). For the other cases (DP1.1 device in the chain, RBR only, 3 lanes faulty of the 4 in the cable), you need 4k30 anyway - and since the only way you could do 4k60 with bad cabling, as opposed to a bad device, is when you have a faulty cable that no longer meets specs, it's probably better to fall back to 4k30 than to try and handle the edge case of a DP 1.4 or later display and video card with a cable that has lost 3 lanes due to a fault, or a cable that's on the edge of electrical stability, and can't quite handle HBR2, but can handle HBR.