On multi-classing over the years:
In AD&D, it was basically cheese, and they attempted to limit the insanity with weird rules on XP gain and arbitrary racial caps.
In 3E/3.5 and pathfinder, it's fundamentally the way you build a character. Single-classed characters are a weird exception.
In 5E, multiclassing is available (it's technically an optional rule) but they moved what were different class paths to be actually "subclass" options inside each class. There exist tailored versions of those like Eldritch Knight or Arcane Trickster which are mean to give some of the multiclass feel within a single class. You can multiclass, but it's generally a big trade-off in high-level power for a bit of versatility. It's fun that it's there, but in most cases sticking to a base class is the best option.
A lot of people who prefer 3.5 or Pathfinder have that preference for exactly that reason. There's infinity possibilities, in a way that 5E doesn't really provide. Personally, I prefer the 5E approach, but I understand why people are attached to the 3.5E system.
We won't talk about 4E here.
