I don't see how disabling the ability to delete a message, helps in this particular case (fear of a possible lawsuit).  Unless you are talking about being sued because someone else destroyed evidence, but I did not think that was your reason; or was it?

AFAIK, neither this nor Yahoo!Groups ever prevented message deletion by the sender.

Isn't there anyone in your group who receives your group's messages by email?  I have 15+ years of my groups' history in my email folders.

If your group is "public", then (some) messages might also be saved forever on the Internet Archive / Wayback Machine.

I have also wondered about giving Moderators the ability to lock other group features (Files, Photos).  But since there is a long history of not doing that, it might not be easy to argue for changing it.

Andy