It's not and will probably never be.

If you want some precedent look at Hono. Initially it was just for the CF Workers runtime (not developed by CF). Then CF started using Hono internally and hired the dev to work on Hono full time. Hono works on any JS runtime.

https://hono.dev/docs/concepts/web-standard

I suppose we’ll see — I’d be surprised if the whole promise of nothing will change don’t hold over the long term. I don’t love that Astro is tied to a narrow set of hosting providers either.