fpurge() was added by Keith Bostic in December 1988 and first appeared
in 4.3BSD-Reno.

Index: fflush.3
===================================================================
RCS file: /cvs/src/lib/libc/stdio/fflush.3,v
diff -u -p -r1.19 fflush.3
--- fflush.3	4 Jun 2025 00:21:20 -0000	1.19
+++ fflush.3	10 Jun 2025 01:13:09 -0000
@@ -126,3 +126,7 @@ The
 .Fn fflush
 function first appeared in
 .At v4 .
+The
+.Fn fpurge
+function first appeared in
+.Bx 4.3 Reno .