For some advanced or special use of Teensy it may be interesting to have custom USB stacks (e.g. one or two USB devices, MSC device etc)
With Teensy3 it is possible to use No USB. Not sure what happens if user selects No USB but implements custom USB stack (I recall vaguely that someone has done this).
With Teensy4 this is not (yet) foreseen.

Suggest to add such option to Teensy 4 and maybe change wording for T3 to "No/Custom USB"

BTW: I' m not saying writing an USB stack is easy, or that the existing one is deficient, but it may be better to keep cores to basic functionality and having developments like MTPDISK_SERIAL or even MIDI_MTPDISK_SERIAL off core.

Also, I do not like to customize cores with my creations for every upgrade of TD.