Hi Jenny,
Well Jenny it's still available but it has possibly been moved to the Hosting Dashboard!
1/. Log On and click profile picture, in the scroll line go to Host.
2/. From the drop down box select the first item, Hosting Dashboard.
3/. Scroll down past 'Notifications' and you will come to Reservations.
4/. Click the required reservation and the booking confirmation will appear with a prominent field...'Message guest' and that will take you straight to the message stream with that guest.
It is probably a good way of going about it Jenny, because, you actually go through the reservation confirmation to get direct to the message stream....so any reservation information you may need is close at hand. If it has changed I think it's a change for the better!
Hopefully it is the same for you in the UK as it is for us in Australia.
Cheers.....Rob