In the past it used to be one giant bar, with the max just getting extended out with each update.
With the release of Battle for Azeroth, each expansion got it’s own skill bar, it’s own progress.
Classic goes to 300
Outland, Northrend, Catalcysm, Pandaria each go up to 75
Draenor and Legion each go up to 100
Kul Tiras or Zandalar depending on your faction goes up to 175 now since the Rise of Azshara update.
This makes catching up to current tiers as you put it a lot easier, you go to the trainer for the expansion you’re in, and you can get started without having to go up through all the old content, some of which may be very hard to come by materials for now.
You can certainly go to the older ones, if you have a gathering profession to match your crafting then you can collect the mats you need yourself and collect all the things