I used /locatebiome Basalt_Delta and it told me to go to a set of coordinates. This area was in loaded chunks from before 20w15a. I tped to the coordinates and tried to find the Basalt Delta. In this previosly loaded area, there is a soul sand valley. TLDR; /locatebiome took me to a preveiously loaded chunk from a different version and told me it was a basalt delta