the authors of any spec or software own the copyright. if you mean license, it looks like the project is under a permissive license: https://github.com/typst/typst/blob/main/LICENSE
So, it appears that the Typst language is not pulished separately from the open source compiler, CLI, etc. Assuming tha the language spec would be essentially the Reference Guide at https://github.com/typst/typst/tree/main/docs/reference
Yep, the "project" shows licensed under Apache 2.0. And, in the readme - they state that they are developing in collaboration with the community. I have been thinking about a blog authoring desktop tool for writing....that would pulish via nostr long form. I see that the project you referenced appears to be focused on a mobile authoring environment.