@Zappa0, I have been experiencing this issue lately too, particularly among instant bookers. Many of my recent instant bookers have gone the entire trip without ever responding to my messages and me never hearing anything from them other than their initial short message when booking.
Recently, I had an issue where someone broke something in my space, so I had to get in touch with the next guest in line to explain the situation to him. I could not get him to respond to my messages on AirBnb, so I went ahead and texted him. He said he had not gotten any of my messages and would check at once.
Come to find out, he had not received any notifications. I didn't ask him because I didn't want to bother him too much, but I'm assuming he hadn't turned on notifications, hadn't checked the email linked to the account or a combination.
I think perhaps many guests are in the same situation, where they are unaware they are receiving the messages, especially if they are newer to AirBnb or infrequent users.