The GeoShapeValue data type has been planned for Wikibase for some time, but its progress has stalled. There has been competing standards and a lack of confidence on which format to use. The lack of such a data type has caused harm to the projects, as this is a basic data type. Since the GeoSPARQL standard was completed in 2011, we have been shown a way forward: use a data type for each underlying format, e.g., one for WKT, one for GML, etc.
Since WKT is the oldest format (being used internally in Shapefiles, I believe, the de fact GIS file format since the 1990s) and simplest, I think we should start with a WKTGeoValue data type. I am not familiar of the complexities of the data model, but it could possibly be a subtype of GeoShapeValue.
Since it would be easy to convert WKT to some other format, there is a way forward. As bad as it would be to deprecate a data type and convert data, it would be worse to prevent the data from being used at all, as is the case now.