That sounds quite conspiratorial. Are you saying the majority of popular content on Substack is actually just "controversial" people rather than quality journalism?

Nobody is arguing that the majority of popular content on YouTube is controversial, but there have definitely been arguments that YouTube radicalizes people.

If they can say that for YouTube, why not for Substack if it implements similar discoverability?

It doesn't have to be the majority of popular content.

If they actively recommend anyone, they will end up recommending people that are controversial to some groups, and they will be attacked as curators and 'platformers' of these people.

They're going to get that anyway for hosting people. And if you read their writing on the subject they do want to improve discovery as well as giving direct support to specific writers: https://blog.substack.com/p/substacks-view-of-content-modera...

"At the same time, while we take a hands-off approach with who may use the platform, we will continue to take an active approach in helping and promoting promising writers. We are doing this by improving discovery on the platform and building programs, such as fellowships, grants, and mentorship, to support writers. Our partnerships team will also continue to work with high-revenue and high-potential writers."

So I'm not sure this claim about avoiding recommendations is actually true.

No. I can't find the original post, but I do remember reading a Substack post (by the founders) saying that they deliberately don't like using content recommendation systems.