Really, I see one right here:

    rank_by_age(items) // Good
    rank_by_age_and_poster_reputation(items) // Probably   
    rank_by_on_topic_ness(items, forum_subject)
    rank_by_likes(items)
    <-- here -->
    rank_by_engagement_likelihood(items) // Bad?
    rank_by_positive_sentiment_toward_clients(items) // Bad

Poster reputation is deterministic. How many times has this poster received positive feedback based on their content?

On-topic-ness is deterministic, if a bit fuzzy. That said I think the likes will reflect this, if you post a thread about cooking potatoes in the gopro subreddit, your post will be downvoted and probably removed via other means in which case it's presence in the feed is already null.

Likes are again, deterministic. How many people upvoted it?

In contrast:

Engagement likelihood is clearly a subjective, theoretical measure. An algorithm is going to parse a database for other posts like this, see how much attention it got, and say "is this likely to drive engagement." That's what I'm talking about.

And positive sentiment towards clients I can't quite read? I'm guessing you're referring to like, community sponsors but I'm not 100% certain. But that almost certainly is a subjective one too, and even if not, it's giving people with money the ability to put their thumb on the scale.

I don't think "deterministic" is the right term to capture this concept. An if-statement which bans posts containing a political phrase would be 100% deterministic, or one which prioritizes anything from a username on a list.

> On-topic-ness is deterministic, if a bit fuzzy.

If you permit that exception (even for good reasons) then it reveals how the original "algorithmic elevation" is too vague and unenforceable.

All someone needs is a ToS footnote like "this forum is provided for truthful international news and engaging with $COMPANY in a positive way." Poof, loophole. Anything the moderator (or moderator-algorithm) decides is "untrue" or "negative" becomes off-topic and can be pushed down.