Same problem! I am a renter, who found a listing that I liked but it then said "This listing in no longer available". Because of this thread, I decided to email Airbnb to check if it was actually available. It was! They wrote:
"Upon investigation, I could see that the Listing is available to book for reservations. Screenshots attached for further clarification.
In case, you are facing any issues in booking the reservation. Please do let me know I will certainly assist you."
After following the link they sent but seeing the same message, I wrote them back and their solution was:
"I request you to try to book any other listing with Airbnb."
As a renter, it's not too big of a deal. But I can imagine that the hosts must be very upset when this happens! This thread has been going for two years and the problem is still going on.