Fedi Admins (or at least StatusNet admins) used to use a mailing list. It's still running on my server, last used in 2013 by about 25-30 instance admins. Looking at the membership is a blast -- some are still active in the #Fediverse, but I don't know if they're still running instances.
Looks like the mailing list fell out of favour, replaced by an IRC/XMPP bridged chatroom.
Anyway, don't use it, I don't think my server can handle the 1000s of admins today.