2021 update: Wikimedia has switched from osm2pgsql to imposm3, support for more relation types can be configured, now type=waterway is supported in addition to previous three types. It should be further reviewed which other relation types can be and should be supported.
T155924: Kartographer's maplink/mapframe ExternalData should render super-relations, T154599: Geoline service not retrieving OSM waterway despite the Wikidata ID, and T155923: Kartographer maplink/mapframe ExternalData should render all geolines and/or geoshapes in an OSM relation regardless of continuity or whatever it is (it's not working properly, basically) are all aspects of how to handle relations other than type=multipolygon, type=route, and type=boundary with way members. Rather than trying to figure out exactly which issue to comment on, I'm merging them all in here.
I don't know if the wikidata tagging in OSM in those issues is correct, but the basic issue is creating a geometry from an arbitrary relation. In the issues referenced, they were waterway relations, boundary relations containing relations, and public transport route masters.
multipolgon, route, and boundary relations have well-established ways of creating geometries from them and are handled by osm2pgsql. Some of the relation types above like waterway relations could be handled by osm2pgsql but are not with the default data transformation. Others, like public_transport and relations of relations cannot be handled by osm2pgsql, and may not even have a sensible representation as a single geometry with tags.
This means if we decide we want to handle arbitrary relations, we would need to deploy another database specific for geoline/geoshape and abandon the possibility of returning OSM tags for some objects. We could then build a GeometryCollection for all relations, which could be used to draw something on a map, but not always a good interpretation. For example an area with a hole drawn as a MP when interpreted this way is a closed line inside a closed line.