Setting dump device from loader(8) has not been supported since 2002.

sys/boot/common/loader.8

@@ -409,13 +409,6 @@ when the kernel is booted.
 This can be overridden by setting
 .Va rootdev
 explicitly.
-.It Va dumpdev
-The name of a device where the kernel can save a crash dump in case
-of a panic.
-This automatically sets the
-.Va kern.dumpdev
-.Xr sysctl 3
-MIB variable.
 .El
 .Pp
 Other variables are used to override kernel tunable parameters.

sys/boot/forth/loader.conf

@@ -73,7 +73,6 @@ module_path="/boot/modules"	# Set the module search path
 #boot_verbose=""	# Causes extra debugging information to be printed
 #init_path="/sbin/init:/sbin/oinit:/sbin/init.bak:/stand/sysinstall"
 			# Sets the list of init candidates
-#dumpdev="ad0s1b"	# Set device for crash dumps
 
 
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