Here's the current state of this feed type:
We did a pretty deep dive into PHP/Composer packages a while back, and our conclusion was that they were very difficult to implement due to the way the tightly integrate with git repositories.
However, we did this assessment without any user partners, and we know next to nothing about PHP, so it could be we misunderstood or looked at the wrong things. Maybe not everyone uses the tight git-repository integration? Hard to say. This is why we partner with customers now.
Since then, there haven’t been too many requests for it, and we have no idea what the level of interest is for. Please add to QA#2690 if you've got some insight.
You're the first person to inquire about it in over two years... but that same document talks about how we partner with users, and I'd encourage you to check out the RPM Thread -- we've got some great user partners in that!