To my humble understanding common use cases for ipv4 igmp on (W)WAN would be services provided by the ISP, .e.g.
Voice over IP (VOIP)
Video on demand (VOD)
Video conferencing
IP television (IPTV)
Upstream ISP node (multicast source) would broadcast whatever service is provided and it would a require a downstream client node (listener) to pick up the broadcast (and make use of it).
Thus, if no such service is being provided by the ISP it would seem logical to disable ipv4 igmp on (W)WAN without impeding general connectivity. Is that a correct assumption or I am missing something?
I have blocked broadcast/multicast at my border router in both directions for years, knowing that no services I knowingly consume or provide required it.
Just as a data point the only ISP service I know of personally that used igmp is my former ISP's VOD/IPTV service, I believe they offer streams of "live" TV and for those IGMP/multicast can be more resource effective than unicast. For xdsl customers of their this offers little comfort, as a multicast stream "eats" just as much access bandwidth on the dsl link than a unicast one would. On FTTH they actually had it set up so that the multicast streams did not eat into the customers' contracted data rates... but I digress.