Unix has a plethora of IPC APIs, almost all of which were invented after signals (e.g. sockets). Signals themselves got new APIs long before signalfd: sigactio

Unix has a plethora of IPC APIs, almost all of which were invented after signals (e.g. sockets). Signals themselves got new APIs long before signalfd: sigaction and posix real-time signals were already a thing, as were posix threads, when Linux was invented.