This is the correct explanation (thank you) There’s no intention to hide the fact that certain links are blocked. The bug is that the status of failed message sends (not limited to blocked links) only shows an exclamation mark against the last message on web.

But it's that silence which deceives the user into thinking that the message never arriving is a technical malfunction and not intentional censoring. How many times, do you think the user would see the censoring message and be cool with it? Maybe once or twice, then they will stop using FB likely forever and tell all their friends. But does FB want that? Hell no, they want to retain the user for data collection and ad showing purposes. So how about just not showing the censoring message, make the user believe there was just a technical glitch and not insidious shadowbanning?

I think this "bug" is intentional and done for plausible deniability in case of a lawsuit: "See, there is transparence. It was just a bug in the code. Human error."

Re your last paragraph. Would that really work though? You subpoena the person who wrote the code and ask them under oath. I wouldn’t perjure myself to protect the company, would you?

How do you know the bug is unintentional?

As good as they are at 99.99% of what they do, almost every time a friend brings an example to me of Facebook doing something malicious, it turns out to be simple implementation error.

So that got winners of the C Underhanded Contest working there?