I've never heard of Typical but the fact they didn't repeat protobuf's sin regarding varint encoding (or use leb128 encoding...) makes me very interested! Thank you for sharing, I'm going to have to give it a spin.
The recent CREL format for ELF also uses the more established LEB128: https://news.ycombinator.com/item?id=41222021
At this point I don't feel like I have a clear opinion about whether PrefixVarint is worth it, compared with LEB128.
But, the thing that tends to tip the scales is the fact that in almost all real world cases, small numbers dominate - as the github thread you linked relates in a comment.
The LEB128 fast-path is a single conditional with no data-dependencies:
if ! (x & 0x80) { x }
It's hard to beat.
if x <= 240 { x }