Another thing I think is fine is "because you watched ____" and suggestions based on that. Like we see a video that the user watched the whole video or liked it, and then we find similar videos to that I don't think its that invasive and may be helpful.
As long as its transparent and the user has control to use it or not I think thats the key. The fediverse however is enormously privacy and corporate sensitive, so I can see that it is a tight line to walk here.
We shouldn't fear these algorithms, the key though is everything has to be under the control of the user. A simple way is just tags, which are widely used almost everywhere. If the user has their panel and can block tags or words or give a rating from +10 to -10 for any keyword or tag, you can accomplish a hell of a lot with no black box fuckery.
