Comment 17 for bug 1718453

At this point, I think it's safe to say that implementing full multiarch support for AppStream simply isn't going to happen, at least not anytime soon. It's simply easier 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 ID conflicts.
Resolving all of these issues is possible, but the huge effort IMHO doesn't justify the gains. If someone else works on this, I'd assist the work, but I will unlikely work on this myself. I originally intended to, but TBH, this issue has really low priority nowadays.

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 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 adding web applications, but may work for the multiarch case as well, at least for a select few of high-profile application packages.