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All colour TV systems have reduced horizontal resolution for the chroma signals. Legacy analogue PAL/NTSC/SECAM, SD-SDI, HD-SDI and 4K. Some digital transmission methods, sometimes known as 4:2:0 as against the usual 4:2:2, reduce vertical resolution likewise. All of this takes advantage of the fact that the eye cannot resolve fine detail in colour the way it can in monochrome.
If the H colour resolution is already halved (even more reduction in analogue) then halving vertical resolution really doesn't matter.
Incidentally, a standard PAL-D decoder moves the chroma half a line down the screen compared to the luma. Again this doesn't show. It matters if you're recovering old PAL material where you would normally be using much better decoders that don't have this effect.
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