You can setup an archive gmail account and have it get everything (from now on) from the group, it'll be there for evidence if needed in the future.
>>> disallowing members from deleting heir own posts
I could be wrong on this (not all that knowledgeable on it) but another reason this won't happen is GDRP, right? Wouldn't the ability of the [mod/group/GIO] to restrict the user's data deletion by the user themselves run afoul of it?
>>> Groups.io doesn't flag moderators/owners when someone deletes a post.
Like J said, you're really asking for having mode message-deleted notification. That message can be part of the evidence, the rest would be the original email from the backup account mentioned above.