What replaced middle management
Removing a layer is easy. The hard part is that its work doesn't disappear with it.
When we removed middle management we inherited its job: context routing, prioritization, and telling people uncomfortable things. Those don't vanish. They redistribute, usually onto whoever is least able to say no.
What replaced it: monthly 1:1s with everyone, bi-annual company-wide retros, a semiannual review of management by the team, and a shared board where clients and numbers live in the open. It's more work than a manager. It's just better work.