A year or two ago they made the change to disallow access to messages from m.facebook.com, so I started using mbasic.facebook.com for messages. Recently they blocked the mbasic.facebook.com entirely for iPhones. It's clear that there reason is to force mobile users to install messenger, because messages are still available on m.facebook.com and mbasic if you visit in a desktop browser.
I already have a FB app on my phone, Instagram, which is more useful for messaging in my circle and less annoying. I refuse to be forced to install their app, so the only response to this is to stop using messenger. Already my messages are going unchecked and will stay that way until I get to a desktop.
The vagaries of finding content you saw minutes before, and the almost entirely random way posts and everything is organized, is frustrating too.