icu-config has been deprecated by its upstream developers for some years now. In icu 63.1, icu-config is optional to install, but is installed by default. In the next version, icu-config will no longer be installed by default.

The developers recommend downstream packages use pkg-config to detect the icu libraries.

Debian (and its derivatives, such as Ubuntu) will remove icu-config from its icu development package (libicu-dev) very soon. Hence, php needs to use pkg-config to detect icu.