diff --git a/contrib/one-true-awk/main.c b/contrib/one-true-awk/main.c
index 4e68bfb46d2..35d0ca7d098 100644
--- a/contrib/one-true-awk/main.c
+++ b/contrib/one-true-awk/main.c
@@ -89,8 +89,10 @@ static const char *
 setfs(char *p)
 {
 	/* wart: t=>\t */
-	if (p[0] == 't' && p[1] == '\0')
+	if (p[0] == 't' && p[1] == '\0') {
+		WARNING("-Ft to imply tab separator is deprecated behavior.");
 		return "\t";
+	}
 	return p;
 }
 
diff --git a/usr.bin/awk/awk.1 b/usr.bin/awk/awk.1
index b85efe9d87c..20bb510a151 100644
--- a/usr.bin/awk/awk.1
+++ b/usr.bin/awk/awk.1
@@ -23,7 +23,7 @@
 .\" THIS SOFTWARE.
 .\"
 .\"	$FreeBSD$
-.Dd $Mdocdate: June 6 2020 $
+.Dd July 30, 2021
 .Dt AWK 1
 .Os
 .Sh NAME
@@ -812,3 +812,11 @@ to it.
 .Pp
 The scope rules for variables in functions are a botch;
 the syntax is worse.
+.Sh DEPRECATED BEHAVIOR
+One True Awk has accpeted
+.Fl Ft
+to mean the same as
+.Fl F\t
+to make it easier to specify tabs as the separator character.
+Upstream One True Awk has deprecated this wart in the name of better
+compatibility with other awk implementations like gawk and mawk.