The formats were originally defined by the Open Geospatial Consortium (OGC) and described in their Simple Feature Access[1] and Well-known text representation of coordinate reference systems[2] specifications.
[4] This WKT format was initially defined by the Open Geospatial Consortium (OGC) in 1999, then extended in 2001.
The updated "Well-known text representation of coordinate reference systems" standard, sometime known as "WKT 2", was adopted by the Open Geospatial Consortium in 2015.
[7] Confusingly, the original 2015 "WKT 2" standard has a version number 1 for the new, stricter WKT-CRS specification.
Those units of measurement have been clarified in the 2001 update and the WKT 2 specification is consistent with that clarification.