As I understand it (and I may well be wrong) in stellar classification the spectra are rectified and this is built into the definition of equivalent width (see The Classification of Stars Jaschek and Jaschek).
The issue of what processing spectra should have depends I think on what scientific purpose the spectra is to be put and to a degree what type of instrument it was captured. BeSS had a particular goal and hence it’s rules.
As an instrumental example short exposures (<10min) I can see no statistical difference between a bias frame and a dark from for my SXVR H694 at -10deg so why use a dark frame and increase the noise why not use just a (constant) bias correction? Similarly for a fiber fed spectrograph where the spectra is always on the same pixels why flat field if you use a standard star for instrumental correction?
Just some random thoughts.