I do wonder if this is just a symptom of monetization. Free advertising with viral posts was possible for talented marketers until the early 2010s. Now you have to pay.

OTOH I have seen examples where messages were supressed. A FB acquaintance was sued under the DMCA for posting data that has since legally been deemed public domain. I suggested setting up in the Netherlands where DMCA is not recognised, via Messenger. Meeting this person in person sometime later, it turned out this message was never delivered. They'd thought I was working for the company that sued them.

The shift was the news feed IMO. You have to consider the profit model. Users are the product.