If we had an open source algorithm for Mastodon/Pixelfed that learned based on the words in the post and image/video we could have a Following + For You feed that showed you all the posts from people you follow and you could choose to see, say, 1 recommended For You post after every 3 posts from your Following feed. With the option to disable For You posts completely or tweak how often you see these.
Discovering new people to follow on mastodon/pixelfed isn’t great (hashtags are rarely used and make posts look ugly) so I still occasionally use twitter because I often discover new indie animators/gamedevs showing off their project making it really nice to browse the For You feed.
That’s not really how most recommendation algorithms work. A good basic algorithm would be showing you content that the people you follow are following or showing popular posts from a certain category that you tend to like