Two other old Yahoo groups I belong to have also had occasional messages posted after the group had moved here. In both a moderator replies fairly quickly, "Sorry! This group has closed. We moved the group -- including archives -- to Groups.io with the same group name. Here is the url- xxx.com. Please send future messages to that group."
What I did, when I moved any of my groups to Groups.io, was leave the Yahoo group set to everyone is moderated. That way, when someone sent a message to the old YG! group, I would delete the message and then email them and make the same explanation as above. Almost without exception, the person would email me back and say it had been a mistake. That they sent the message from their email and the auto-address had filled in the wrong url. They'd also usually then delete the Yahoo address from their email address book.
tommy0421
Tommy Meehan