This is something I use Corel Paintshop Pro X4 for. The ability to remove JPEG Artifacts from images by first adding subtle noise and then selecting an edge preserving smooth. This removes the blocky look to highly compressed images. Interestingly the Despeckle feature did it in one step on the first Photoshop Elements app.
I'd like to see this feature in Affinity Photo in the future. Maybe under an image repair persona,