According to https://docs.inedo.com/docs/proget/feeds/other-types the Composer feed type is planned for a maintenance release of ProGet 2024, which is great news!
We're currently using Satis for PHP packages alongside ProGet (for NuGet, npm, Python). Working with Satis is a bit cumbersome, that's why we would very much prefer using ProGet for PHP packages, too.
Satis downloads the distribution zips for all packages and generates a static site with package metadata and the zips. I guess ProGet will handle Composer feeds in a similar fashion?
Feel free to reach out to me via email if you need further feedback or beta testers for the Composer feed type.