I wanted to give everyone an brief update on this issue, which I believe is fixed (but am working on resolving the underlying problem). I'm working up a blog post with more technical detail on exactly what happened, but I believe the immediate issue has been resolved.
In brief, there were a few things that contributed to the mass-removal of folks. All mail that goes through Google servers (gmail and business domains) uses an IPv6 security policy that rejects mail without a IPv6 reverse lookup. To get around this, we had a Postfix filter in place that read the rejection message from Google and retried via the server's IPv4 address. This was working fine until some time earlier this month when Google changed the wording in the message, causing messages to start to bounce. The listserv software keeps tracks of bounces and started unsubscribing folks to keep off of spam lists.
Thanks a lot to the folks in the code4lib Slack (Jeff in particular) for helping debug the issue and get a fix in place!