Instead of waiting, hoping that big companies will implement the standard. Just make it as easy as possible to adopt by having receivers for all platforms and making client libraries that can cast to AirPlay, Chromecast, FCast and others seamlessly.

Doest current app support AirPlay and Chromecast as different receivers/backends? Websites doesn't mention anything about it. Is there any plan also for iOS app?

Some feedback: I would also add dedicated buttons for downloading MacOS and Windows binaries - for typical users gitlab button will be to scary. Website also not clear if there is and SDK for developers (the one that supports also airplay and chromecast) and what languages bindings it supports.

The plan is that the FCast SDK will eventually allow you to cast (with a reduced feature set compared to FCast) to Chromecast, Airplay and others. Improving the download experience for users for sure is something we will look at.

GitHub has package repos for hosting package downloads.

A SLSA Builder or Generator can sign packages and container images with sigstore/cosign.

It's probably also possible to build and sign a repo metadata index with GitHub release attachment URLs and host that on GitHub Pages, but at scale to host releases you need a CDN and release signing keys to sign the repo metadata, and clients that update only when the release attachment signature matches the per-release per-platform key; but the app store does that for you