Do you know of anyone working on Majordomo?
Of course not. Both MJ1 and MJ2 have been abandonware for a long time.
(This has been the only mailing list software that has proven to be capable of handling the mailing list traffic for the kernel.org mailing lists.)
This suggests it's been quite a while since you looked at the alternatives, since MJ1 is not particularly fast. The most popular actively maintained open source list managers these days seem to be Mailman (python and postgres, from the GNU crowd) and Sympa (perl and mysql, from Renater in France.) Both are used to manage large numbers of large busy lists, with archives.
I switched from MJ1 to MJ2 in about 2002 because MJ2 had much better list management and filtering tools, but finally gave up on MJ2 a year ago when it had trouble with perl 5.20 and moved everything to Sympa without much trouble.
More to the point, DMARC isn't going away. If ARC works, which seems likely, and your lists put ARC headers on the mail, your lists can keep working the way they do now. The alternative is either gross hacks to rewrite From: lines, or to make your lists irrelevant by telling people they can only use non-DMARC mail providers which in a few years will be none of the large ones.
Furthermore, having actually added anti-DMARC shims to both MJ2 and Sympa, I know that you don't have to change the list manager code at all other than one line to call the shim rather than calling sendmail (or whatever) directly. The shim does what it needs to do and passes the message to the real mail injection program. My current shim which does the dmarc.fail From: hackery and adds DKIM signatures is 282 lines of not particularly tightly written perl.
If a bunch of Linux kernel hackers can't do that to MJ1, Linux is in even worse shape than I thought.
R's, John