Nonetheless, here are some I subscribe to:
Big, by Matt Stoller (about monopolies): https://mattstoller.substack.com/
Margins (about business and tech): https://themargins.substack.com/
Flow State (music): https://flowstate.substack.com/
Normcore tech (tech, data science): https://vicki.substack.com/
The Passion Economy (business): https://passion.substack.com/
I also think Substack's UI and UX should be much better and also their search feature is rather primitive, search always should be robust in this kind of businesses.
That said, these are some pretty neat newsletters so now I do find myself wanting to discover more of them. I see an opportunity for something like one of those "Awesome list" Github repos that lists neat Substack newsletters people have found. And honestly that kind of feels more "wholesome" to me than official curation since it's communicating one person's taste (or a curated group's taste) rather than introducing potential incentives for newsletter writers to try to game some system for publicity. This seems like a good start. ;)