I don’t see prebiotics, probiotics, or marigold in the list of ingredients on my feed. I’m not sure how accurate your statement is that most feeds contain them. That may depend on what kind of feed you are buying or where it comes from. I don’t see it but a different feed store might have feed where that is pretty standard. We all see different things at different stores.
Marigolds will make the egg yolks darker. That doesn’t mean they are any healthier, just that they are darker. Several foods will cause the yolks to be darker. Many green leafy vegetables will do that.
Are pre and probiotics necessary? Not really. They won’t hurt and might help, but your chickens are not going to die if they are not in the feed. I purposely feed dirt from the run where the older chickens have been pooping to get whatever probiotics the older birds have into the chicks. That also provides grit and helps get them on the track to developing flock immunities. But my first chicks when I started the flock did not get any probiotics and did fine. I consider it more of a marketing ploy than really necessary. But it will not hurt.