Is there a good reason to continue deploying newer snapshots, if there's already a released artifact of the same version?
Concretely: There's already xyz-1.0
There's projects continuing to have xyz-1.0-SNAPSHOT dependencies.
It appears wrong to me, but maybe there is some reason behind this?
If 1.0 has already been released, the snapshot should not be built any longer and you should probably remove the snapshot from your repository. If new development starts, it should be on 1.1-SNAPSHOT (or 1.0.1-SNAPSHOT, however you decide to do your versioning).
The only time that a project should be deploying snapshots after a release version has been deployed is if it were an alpha, beta, milestone or release candidate.
As you have suggested the projects in question are doing things wrong. After the xyz-1.0 release the SNAPSHOT versions should be updated to the next development version, 1.0.1, 1.1, or 2.0.