Hello,
I’m seeking information about the spectrum of DICOM versions that 3D Slicer can effectively accommodate. Specifically, I’d like to know which DICOM versions are supported on versions 5.4.0 and 5.5.0 of the software.
Appreciate your assistance! 
Considering that Slicer 5.4.0 was released 4 days ago, there are no differences between the latest Slicer Stable release (5.4.0) and the current Preview release (Slicer 5.5.0).
Reading through the following page should help understand which modalities are supported. See Data Loading and Saving — 3D Slicer documentation
In general, yes, we try to support the aspects of the standard that are the most important to the users and developers of Slicer. Unless there’s a demonstrated need we don’t try to implement everything in the standard.