@Lisa5676 You can contact past guests by going back to your message stream with them when they stayed. Then just send them a new message.
I don't think there's any way you can mass message them, you'd have to create a saved message, and insert into the message fields one by one. Certainly more time consuming than sending one email with recipients CCed.
I have a little book where I write down all guests' phone numbers, and personal email addresses if we have exchanged them. I write down their phone numbers as soon as I get a booking, in case there's a glitch on the platform. It's also good practice in case you are ever unlucky enough to have Airbnb suspend your account, in which case you wouldn't be able to contact upcoming guests through the platform.
And if you had personal email contact with past guests and didn't delete those emails, you should be able to find them somewhere in your email inbox or sent box. I have an email folder where I move all email communication with guests so I can find it easily. I don't save every email communication, but at least one from each guest so I have their email address on file.
I'm referring to their personal email addresses here, not the coded one Airbnb assigns. I'm not sure, but I think those coded email addresses are generated for each individual booking, not a constant, attached to a profile.