Index: bin/df/df.c
===================================================================
--- bin/df/df.c
+++ bin/df/df.c
@@ -247,6 +247,8 @@
 #ifdef MOUNT_CHAR_DEVS
 		} else if (S_ISCHR(stbuf.st_mode)) {
 			if ((mntpt = getmntpt(*argv)) == NULL) {
+				xo_warnx(
+				    "df on unmounted devices is deprecated");
 				mdev.fspec = *argv;
 				mntpath = strdup("/tmp/df.XXXXXX");
 				if (mntpath == NULL) {