currently, it seems that when a document gets depublished, a version is created. However, it seems better to not create a version when there is no unpublished version. Namely for example consider:
1) You have bootstrapped live documents (suppose English)
2) You create a translation of such a live document (suppose French)
then the translated document will be created from the 'live' English document, and directly depublished. It doesn't make sense in that case, that there is directly created a version of the French document, which is the content of the English one.
In general, creating a version on depublish only makes sense when there is a unpublished variant already