Not specific to FB, but engagement is not the only thing a platform can optimize for, if a company wants to maximize revenue. Optimize the AI to avoid incentives for government regulation or lawsuits, or just to appeal to a country's regime, individual politicians or lobbyist groups can have huge impact on profits.
"We really want to operate in your country, but we don't want to be seen as submitting to your, umm.. particular netiquette guidance. Shall we design some policies together, that match both our interests?"
Beautifully said, this is a big elephant in the room.
There is a preliminary problem: the mission. "Intellectual curiosity", "Stay for the empathy", "Truth finding", "Thank you for sharing", "Ask the expert", "Feel the outrage", "Access knowledge", "Take this latest"¹ (and of course, "Be addicted!") [²] - these do not bring to the same place. And the wealth in one may conceal that you are in the wrong place.
(¹ "Take this latest": with reference to implementations that show the recent but irremediably hide the past. Like press under some dictatorships. Because if you were interested in Habermas' "chirps" on the events of May 2013, and to read Ferguson's interventions like a book, but you are supposed to be only interested in yesterday...)
Edit:
² I forgot a big one, implicit: "Build peer pressure by discovering others with a similar comfort zone". "...Today a page, tomorrow the world".
Generally what is perceived as shadow banning is just algorithms learning that the poster has low engagement, so it ranks their posted content (comments, posts) lower in the future.