> Which makes me curious... Does anyone here happen to know when #if was introduced in C?

From what I can gather from studying the TUHS archives, the earliest
cpp that supports #if is in PWB:
http://minnie.tuhs.org/cgi-bin/utree.pl?file=PWB1/sys/c/c/cpp.c

Before that, the pre-processor resides within the cc.c command and #if
is not supported:
http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/s1/cc.c