Was a disassembler ever considered part of the standard binary objects toolkit with the assembler, linker, etc. or was that the sort of thing that was more niche and therefore just kinda cropped up when/if someone decided to write one?

There was a Vax decompiler around the time of 4.2BSD that was available from some university (Utah?). It was commonly used to decompile Peter Langston's empire program so it could be run on a bunch of the other Unix minicomputers that were available at that time.  (E.g., we ran it on Pyramid's)