> blocky compression artifacts that show up at 720p

It's just the shitty compression, not the mark of 720p.

This is extremely evident on YouTube now, for a couple of years if the source content is 4K native, they downgrade it to 1080p (so you can stream it with a comfortable bitrate), but despite a good enough resolution (1080p! 2K!) and the need to just downgrade the resolution (yes, double re-encoding, but) the 2K version looks abysmal. Especially evident on live videos with a contrast lights.