dhcpcd integration doesn't support dhcpcd 9.0
dhcpcd integration doesn't work with dhcpcd 9, even when the privilege separation feature is disabled. There are (at least) two relevant changes:
https://github.com/rsmarples/dhcpcd/commit/3063ebb6c8ac7c96196fa923cdd5f7c0384de23b rewrites the daemonization process, which makes NM unable to communicate with dhcpcd.
https://github.com/rsmarples/dhcpcd/commit/b75c7a3fb31198da2b983a58220e2fe163a66eb1 changes the PID file location from run/dhcpcd-%I.pid to run/dhcpcd/%I.pid