I should've written "... that don't just use ASCII characters instead of Hanzi." Of course domains using numbers or English words aren't Pinyin, but the point is that they don't require Punycode to represent.

Why is 1 = yāo? It is yi isn’t it?

一 yi (tone varies based on phonological context) is the ordinary word for 1.

幺 yāo is a common word for 1 when vocalizing a stream of digits. (Such as a phone number / credit card number / account number / etc.) I am told that this originated as a way to easily distinguish 1 from 7 (七 qī) when communicating over a low-quality connection. In the presence of static, yi and qi could sound similar.

CC-CEDICT has a good gloss ( https://www.mdbg.net/chinese/dictionary?page=worddict&wdrst=... ):

> one (unambiguous spoken form when spelling out numbers, esp. on telephone or in military)

Thanks!