Google Research recently committed to help implementing it, in Rust, for inclusion in Firefox.
https://github.com/mozilla/standards-positions/pull/1064: “the team at Google has agreed to apply their subject matter expertise to build a safe, performant, compact, and compatible JPEG-XL decoder in Rust, and integrate this decoder into Firefox. If they successfully contribute an implementation that satisfies these properties and meets our normal production requirements, we would ship it.”