Airbnb has recently removed the "message" button and link from Reservation Request emails. There used to be two options – 1) Accept/Decline, or 2) Message. The latter is now gone. You can only Accept/Decline.
Support informed me that this is not a bug, it's a new "improvement". Hosts are now required to go to the hosting inbox and use a message thread there in order to message the potential guest.
So now instead of one step to message a guest we have to go through several steps, whether on desktop or mobile app. This change makes things just that little bit more difficult and complex for hosts.
I'm struggling to find a positive reason for this change -- perhaps Airbnb wants hosts to "accept" or "decline" without any discussion. But hosts often need to exchange some messages about the reservation first, before we accept or decline. We only offer long-term reservations of 30-days or more, so we always have to message with guests first.