The problem with PS/2 is that most motherboards today don’t have it anymore so you can’t use passthrough adapters.
About two thirds of all current motherboards have PS/2.
Also the last gaming keyboard which still had native PS/2 support was the old non-macro enabled CM quickfire.
Despite Ryzen AMD is still negligible as far as market share goes.
https://segmentnext.com/2018/05/23/amd-cpu-market-share-46/
https://wccftech.com/intel-coffee-lake-amd-ryzen-cpu-market-...
edit: Also they do have a Z370 board with PS/2, the ROG STRIX Z370-G GAMING.
It looks like you got the only mono from Asus that comes with a PS/2 port other than the Apex: https://www.anandtech.com/show/11860/z370-motherboards-asus-...
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N...
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&I...
https://geizhals.de/?cat=mbp4_1151v2&xf=317_Z370%7E7088_PS%2... (I don't know a similar English/US site)