Lots of interesting things are going on in this thread..
Here are my thoughts, which are based on the current typical US training programs.
Decompression diving used to be a regular part of the curriculum in recreational "Advanced" classes years ago because it was taken for granted that anyone diving should be capable of safely executing a dive plan that includes decompression diving. During the "Advanced" class that I took in 1982, as part of the course we executed a decompression dive off the wall of Roatan, and this was nothing that unusual.
However, if you look at the basic entry level course back then, which was the gateway to all things diving, it typically lasted 4 weeks long with 2-3 hour sessions multiple times a week, incorporating roughly ~40+ contact hours BEFORE check-out dives. The advanced class that I took in 1982 also lasted an additional 4 weeks and included advanced physics, physiology, environment, etc. with practical application. The material I was required to learn to earn my "advanced" card in 1982 is now typically reserved for leadership courses (Divemaster through most agencies, or Master Scuba Diver through NAUI).
Fast forward to today, most recreational divers earn their beginning certification in 3 or 4 days. They may have some self-study (e-learning, books, etc) but their academic and pool time probably totals up to about 12 contact hours before check-out dives. A modern "advanced" class is typically 6 additional experience dives, run over a 2 or 3 day course, with little additional academics. The modern approach favors smaller, more easily digestible bites, over multiple additional "specialty courses" as opposed to intensive/immersive all-encompassing programs.
I'm not trying to cast judgment on the pros and cons of the old school approach versus the modern approach. While I think the old school approach made well rounded divers, it was not really necessary to go through all of that for the person that simply wants to go do a couple of 60' reef dives while on vacation in the Caribbean. Additionally, the once a year diver is likely to forget how many pounds of lift a bag that displaces 1 cubic feet of water can provide in a lake (nor are they likely to need that information), so why waste their time cramming it down?
Some agencies and instructors still hold on to the "old school" approach and there's nothing wrong with that. I taught scuba for a total of 10 years between 1996 and 2016 in a university program that was very old school and I believe that the divers that came out of those university courses were substantially better rounded than the ones I've either seen, or grudgingly taught through a dive shop that's now defunct, but that is because we had an advantage of a full semester for an OW course.
The trade-off between the two schools of training is that a modern "advanced" diver is not sufficiently prepared for decompression diving. In reality, the modern advanced diver is a beginner with training wheels that needs to go and gain additional experience in conditions that are similar that in which they've trained. Different people progress at different paces, but generally speaking, a person interested in decompression diving should have, at a minimum, 50 dives under their belt, and I would go so far as to say 25 of those dives should be in the gear configuration they plan to use (doubles or sidemount).
My recommendations for the person interested in decompression diving would be to:
1. Take RESCUE. Yes, it's true that you'll never wake up on a Saturday morning and say "I want to go RESCUE Diving today!" but the course helps you to identify things that can potentially go wrong and then avoid them so you don't need to do a rescue.
2. Take either an Intro to Tech, Sidemount, or Fundamentals course, or if they're in the Southeast US or Mexico I would suggest taking a Cavern course (or maybe a basic cave course).
4. Gain some experience. Go diving and hone those skills. Then take a deco course.
Ken