I cannot think of any examples off the top of my head but I remember there being some games that had a difficulty slider that would just add more enemies (usually one or two) as you made it more difficult. No real game balancing to it. Some areas were hard enough to jump from platform to platform without an enemy but now you can add enemies to make it even harder.

Others definitely just adjusted stats we as gamers could not see. Attack power, defense, etc. Often this was without much game balancing. Like they only tweaked the default setting and left it to fans to adjust up or down from there.

I prefer the Metroid method - that way too hard? Go back and try another direction. Sadly, that requires planning and playtesting and balancing to get right.