How to add true backwards compatibility to 1.16.5 and older versions?

Hey! I'm using via backwards and it really works well, but when people go below y 0, get bugged and can't play well.
and how about the new blocks and items. because i have a 1.20.1 server but most of my players using 1.12.2 like 4 from 6. Can you think of any way to improve compatibility or fix that problem? Thx.
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