There's a public RFC, but it's by no means usable as a bitstream spec. There's an official text document describing the bitstream format (several hundreds pages), but it's under NDA. We use it at work, and getting an updated version is far more trouble than downloading a file on a public server. The document in itself is nearly worthless, though, as it's far from complete. In the end, you still need to dig into libvpx to understand how things work.

I could only find an "overview", not a full spec: https://tools.ietf.org/html/draft-grange-vp9-bitstream-00

I have never seen a spec, do you have a link?

This, perhaps? https://tools.ietf.org/html/draft-grange-vp9-bitstream-00

It's only a draft, and two years out of date, but it is public.

It's a good overview of VP9, but it's certainly not enough to implement a decoder, or call it a "spec".