ICAO adopted the current airspace classification in 1990. The US followed suit in 1993.
But, I found this:
In case you didn't know, the U.S. had 20 different types of airspace designations prior to 1993. (VATSIM)
- Were there really 20 classes?
- What were they and their functions?
- And if possible, how did they compare with the rest of the world pre-1990 in a general sense?
What I could find was the Oct 1993 issue of Flying Magazine. From which, I couldn't find the 20 classifications, just 7 old names and some changes.
Also via here we find: Transition Areas, Continental Control Areas, and Control Areas. All three are now under Class E. That brings the total to 10 out of 20.
- Thanks to @Pondlife for the FAA document and for checking the AOPA magazine article.