I don't see how it could be Airbnb's fault. How could they have possibly known that Gmail's anti-spam algorithm would put their message in your spam folder?

It seems pretty dumb of Gmail to put an email from a domain which you've accepted messages from repeatedly in the past into your spam folder. If you would have thought that messages from Airbnb were spam, you would have marked them as spam a long time ago, not kept on reading them.

But ultimately, if you use an email service that has spam filtering, you should be checking your spam folder occasionally for important messages.