Are forums like HN not also a social medium?
On a message board, 1) I don't have a profile that's rich with personal information, 2) I don't have a dedicated wall for my musings, 3) I don't have an individually-curated timeline, and 4) I don't have first-class social connections embedded in the platform.
Point 1 means that there is much less ability to identify me for the purposes of advertising, which avoids much of the perverse incentive that comes with monetizing social media users.
Point 2 means that I have much less personal attachment to this place as an outlet for creative self-expression, which helps to defuse both a sense of toxic entitlement that I might feel on behalf of the platform providers, as well as the sunk-cost fallacy that might keep me active here even if I no longer experienced pleasure from being here.
Point 3 keeps filter bubbles from fractally proliferating; there is still one bubble, but it's the bubble that everyone else on the platform inhabits.
Point 4 provides a mixture of all of the above benefits.
Again, this isn't to say that message boards are perfect or that social media must be inherently bad, but IMO the differences are important.
The details you list seem incidental to the social media of today. Reddit fits much of what you say but most people would classify Reddit as clearly social media even though Reddit is closer to HN than FB by this divvying of conceptual boundaries. Perhaps the social media of tomorrow involves no wall and meetings in Oculus land. Then we would be talking about how social media is psychologically or socially problematic because of 3D immersion.
IMO the easiest bright line between social media and "something else" media is that social media is populated with content by amateurs or indie producers. If FB became 100% business then it would lose its credentials as "social" media and simply become "traditional" media, notwithstanding any timeline, wall, bubble or heuristic curation. If YouTube became all professionals then it would just become HBO, regardless of whether there are subscriptions, notifications, or channels.
As for the "indie producer" aspect, that's certainly one useful property to consider, but I don't think it's sufficient since pre-internet we had things like 'zine culture which were the bastion of indies, and I would find it a stretch to call zines a form of social media, rather than just indie media.
Like HN bans, Facebook/Google bans wouldn't really be too much of a problem if they weren't _the_ platform in their respective domains. As much as I advocate for alternative platforms like Mastodon and PeerTube, them being a viable alternative is a future I hope to see, not the present reality.
So long as the site acts only as the big room in which the party happens, it's benign. Once the room becomes an active participant and manipulator it becomes what is now a modern "social networking" site, and should be regarded as poison. At that point, leave for your own sake.
Hacker News is one of the most aggressively moderated forums you'll ever come across, the room is very much an active participant and always has been.
And the entire purpose of the karma system (particularly the censorship of downvoted items) is to suggest and promote some content over others. That HN doesn't use machine learning is just a quibble about complexity.
Flagging/purging content is to me more acceptable than manipulation via suggesting/promoting, though, especially if the moderators are members. Maybe making this distinction is hair-splitting, though.
Shadow banning, on the other hand, is a disgusting technique. That seems like more a childish prank than a means of moderation, and only invites negativity (which is what was trying to be avoided, right?).
Overall, it's a pretty reliable source of links to good articles -- and some discussions.
If not, what would they be?
So, no, under the old def HN was never about becoming trillionaires by selling our accts to google or whatever. And under the new definition HN has intense political leanings but is still primarily technical in nature and doesn't censor as much as Reddit or other fundamentalist or inquisition like as sites.