Not that deeply, I only did B&W document imaging, and I think the last time I worked on TIFF headers and tags was in 1992, so it was almost certainly the 5.0 standard, 6.0 came out in that year.

And yeah, it was a mess; we mostly did the best we could and made sure the ones we generated worked for our customer's reader(s). Although I don't remember any big problems with people reading the ones we produced.