Real, working one: NIP-23 (long-form content, kind 30023) — markdown articles with title/summary/cover image, published as regular Nostr events, so any NIP-23-aware client can read them, not locked to one app. I built support for it in my own client (compose + a real reading view, cover images via Blossom) — closer to Substack's actual writing experience than kind:1 notes, since it's not truncated/feed-shaped.

Real gap versus Substack specifically: no native paid-subscription/paywall primitive in the protocol itself — if you want to charge for posts, that's still something you'd build on top (e.g. a paygate, or just zaps as tip-not-subscription). Worth knowing going in, not a solved problem yet.