Seems like it was only in force for a day, and carried no penalties. Also seems like it was aimed at people who are looking at their smartphones primarily (i.e. playing Pokémon GO).
I don't think they were looking to cure the menace of 歩きスマホ in the stroke of a pen, but rather wanted awareness of the issues of being engaged primarily with your phone rather than your environment.
https://www.asahi.com/articles/ASN71728GN71ULOB004.html#:~:t...
The language chosen for defining 歩きスマホ (“walking smartphone”) uses the word https://jisho.org/word/注視 in its definition, not sure if this is just formal language, or there is some important distinction between 注視 and 見て here.
Nonetheless, here is the draft:
http://www.city.yamato.lg.jp/web/content/000159510.pdf
And if you happen to live in Yamato, and want to track this, here is one of the official pages for tracking the ordinance.