As best I can see that assigns a Content-Type ("MIME type"), distinct from a Content-Encoding supported by browsers.

So it specifies what Content-Type header your browser should use for a .zst file, but doesn't represent a plan to let your HTML, JS, CSS etc. be delivered compressed using Zstd and transparently decompressed by the browser, as is possible with the gzip and br Content-Encodings.

(But turns out pauldino's comment points out it is in the "content coding registry" at https://www.iana.org/assignments/http-parameters/http-parame...)