Hello everyone and I'm sorry for the late reply to your questions regarding the WPS push button.
Android 8.0 included included the feature Wi-Fi Protected Setup (WPS Push Button) that could be used to connect WPS-supported client devices, such as wireless printers, to your router wirelessly. This feature has been removed by Google in Android 9.0.
I hope that answers all the questions about this feature and why you aren't able to find it.
