Maybe someone who understands the specs better can add more information, but as far as I can tell from Wikipedia, USB4 supports DP in two different ways: tunneling and alternate mode.
When tunneling, you are limited to DP 1.4a and wouldn't be able to do 8k/60Hz without compression (which is also optional, I believe). In alternate mode, which essentially runs DP over the USB cable without wrapping the protocol, you can use DP 2.0 and should be able to do 8k/60Hz. 8k/60Hz requires more than 25.92 Gbit/s (which is what you get in DP 1.4 HBR3, 32.40 Gbit/s including the encoding symbols), so I assume the alternate mode is not actually limited to the 40 Gbit/s bandwidth of USB4. Because it is unidirectional, I believe it can use up to 80 GBit/s.
It looks like Thunderbolt 3 only supports DP 1.4 (optionally, only 1.2 is mandatory), and you'd be limited to the same speeds as tunneling in USB4.
Honestly, if we are confused about this then I have no idea how the general public can be expected to know what their ports are capable of.
Compression support (DSC) is mandatory since DP 2.0. So running DisplayPort Alt Mode 2.0 (which is basically "DP 2.0 over USB-C") should guarantee DSC support.
The new USB4 Version 2 also bumps tunneled Displayport to the latest DisplayPort 2.1 spec. Then you can tunnel up-to 80 GBit/s as well (without using alt-mode).
Thunderbolt 3 is a funny protocol, because in reality only Intel made PHYs for it, so the DP version is bound to whatever Intel controller you've got. The 6000 series controllers have DP 1.2, the 7000 series has DP 1.4 and DSC.
When tunneling, you are limited to DP 1.4a and wouldn't be able to do 8k/60Hz without compression (which is also optional, I believe). In alternate mode, which essentially runs DP over the USB cable without wrapping the protocol, you can use DP 2.0 and should be able to do 8k/60Hz. 8k/60Hz requires more than 25.92 Gbit/s (which is what you get in DP 1.4 HBR3, 32.40 Gbit/s including the encoding symbols), so I assume the alternate mode is not actually limited to the 40 Gbit/s bandwidth of USB4. Because it is unidirectional, I believe it can use up to 80 GBit/s.
It looks like Thunderbolt 3 only supports DP 1.4 (optionally, only 1.2 is mandatory), and you'd be limited to the same speeds as tunneling in USB4.
Honestly, if we are confused about this then I have no idea how the general public can be expected to know what their ports are capable of.