I remember in 3e we came across a crypt full of mummies. Mummies in 3e are kind of nasty. An aura of fear that causes paralysis. Mummy rot. Everyone was really worried, and then we realized we had a paladin. Paladins in 3e are immune to fear, and immune to disease, including supernatural and magical diseases. In 3e, mummies are just tough zombies to a paladin.
Agreed, that is why a redid them: dragons
I like combat to be tense, exciting and allow all the players to meaningfully contribute.A general trend in 5E monster design (probably applies to 3E and 4E as well) is the reduction of asymmetric monsters. Monsters with immunities, or special attacks or special requirements existed to provide an alternative approach. Some monsters can't be assaulted directly and need to be handled a different way. These 'puzzle' monsters exist to force players to look at other tactics.
Just reducing CR doesn't make a monster "nerfed".
To be called "nerfed" a monster should be worth less than its CR.
If you think the Will 'o the wisps are weak, just combine them with a Banshee... [emoji6]
Something that does earn the epithet "tragically nerfed" is most MM demons. The Nalfeshnee and the Marilith come to mind.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.