`ln -s` inside WSL *does not* create Windows symbolic links; they are only made with mklink in cmd.

link.sh

#!/usr/bin/env bash

variant="${1:-retail}"
wowdir="/mnt/c/World of Warcraft/_${variant}_/Interface/Addons"
if ! wowdirWIN=$(wslpath -w "${wowdir}") ; then
	echo "failed to find wowdir for ${variant}"
	exit 1
fi

# TODO: magic filters to only link addons that apply to the current variant?

mklink () {
	if [ ! $# -eq 2 ]
	then
		echo "mklink requires two arguments"
		exit 1
	fi
	# mklink /D Destination Source
	# echo "cmd.exe /c \"mklink /D \"$1\" \"$2\"\""
	cmd.exe /c "mklink /D "$1" "$2""
}

for i in ./* ; do
	if [ -d "$i" ]; then
		addon=$(basename "$i")
		if [ -e "$wowdir/$addon" ] ; then
			echo "$addon already exists"
		else
			mklink "$wowdirWIN\\$addon" "$(wslpath -aw $addon)"
			#exit 0
		fi
	fi
done

Yes, this was still more appealing to me than writing a batch file.