I remember when Chuck Forsberg died, the author of YMODEM, nearly 10 years ago now:
https://news.ycombinator.com/item?id=10593617
The XYZ Modems: https://web.cecs.pdx.edu/~rootd/catdoc/guide/TheGuide_226.ht...
As far as I can recall, it didn't have a sliding window, once protocols, like kermit, added sliding windows the speed jumped a huge amount.