More anecdotal evidence.
Probably the best place to know what is going on with multicast is on
the mboned list (yea that still exists):
<https://mailarchive.ietf.org/arch/browse/mboned/>
Second best might be the Internet2 community where a number of
institutions that have always had it might still have it turned on.
Though there has been only one post in all of 2018 on their list if
that tells you anything.
There is almost no public multicast monitoring anymore. Practically
all dead. You could poke around the I2 router proxies and see what is
happening on some of the last of the remaining IP multicast-enabled
Internet (hint, mostly noise from odd devices that have strayed outside
the local institution):
show multicast routes
show pim join
...
I keep hearing about some research project in the deep blue sea that
uses it. You might find a small amount of it out there, but I'd bet
overall its usage is teeny tiny and limited to a shrinking number of
networks. Turned off interdomain multicast when I returned here in
2016.
John