I understand Bob's concern. What he's looking for is a way to preserve evidence of slander or liable. Whether to protect the group or the author is irrelevant. Unfortunately taking away the right of a person to delete their own written works runs afoul of copyright law because it because it puts the group owner (as the person ultimately responsible for the group) in the position of taking ownership of the message (in essence, the copyright) away from the author. As someone else noted, it most likely also runs afoul of GRDP. This is why groups.io feels it cannot restrict a message owner from deleting their own written works.
I suspect the best way to preserve such evidence is to maintain a separate membership to receive and save all the messages from a group, perhaps as full featured digests, as formatting can influence how a message is understood. These can be held should some need arise. I think it might also be advisable to let all the members know, perhaps as a Special Notice so everyone sees it, as it may help members to be more circumspect about what they post.
an original in a screenshot of it on groups.io would be more desirable for possible defenseI'm just not seeing that, nor am I seeing a need for a defense. It's gone from your site. Over and done.
Regarding keeping tabs on members' actions, why not request an optional notification for post deletions? I still completely fail to see how disallowing members from deleting heir own posts even comes close to solving your problem. And on top of that, surely there are not a large number of people in your group actually perpetrating this unique kind of offense anyway?