Comment 25 for bug 1564570

At this point, I think it's safe to say that implementing full multiarch support for AppStream simply isn't going to happen. It's simply better for app authors to provide native implementations for amd64 instead of relying on i386 compatibility.
If support was implemented fully, we would download lots of additional big metadata and run into all kinds of troubles of which app gets selected when there are conflicts.
Resolving all of these issues is possible, but the huge effort IMHO doesn't justify the gains.

There is a workaround for high-profile packages that are only available on one architecture: *If* i386 support is enabled by default, the appstream-generator tool can be instructed to inject metadata for a specific package directly into the final metadata collection. See https://github.com/ximion/appstream-generator/blob/master/docs/asgen-config.md#injecting-extra-metainfo--removing-components for details. This is intended to be used for web applications, but may work for this case as well. Steam may be important enough for that exception. The final decision is up to Ubuntu and the metadata generator admins though.