Apple deprecated URI schemes as an app interop scheme in favor of Universal Links, where an app claims ownership of a portion of https:// URL space (e.g. Google Maps would claim https://google.com/maps by via https://google.com/apple-app-site-association). A major reason for the change was that sneaky AdTech tracking SDKs were surreptitiously probing for whether URL schemes were supported to detect what apps you have installed on your phone, for nefarious fingerprinting purposes. Unlike custom URL schemes, Universal Links fall back to the web version so there is no reason to provide an API to see if they are supported by a native app or not. Apple still allows apps to use custom URL schemes to integrate with other apps, but only a limited number of allowlisted schemes that must be statically predeclared in the app's Info.plist metadata submitted at app review time. WhatsApp is big enough to make the cut, but they also support Universal Links via the wa.me domain and https://wa.me/apple-app-site-association).