Do you prefer classes to have niche protection like in TSR D&D where one class may shine in one pillar over the others, but not in another. Or do you prefer classes to all be equally able to contribute the same in all three pillars like in later editions?
This question can't be meaningfully answered. Your definitions are too terse to be complete. I have no idea what scope you're looking at, and so I have no idea what you're even asking. If such a thing exists, this is a "not even wrong" territory of game design.
In other words, I reject the assertion that "niche protection" means "one class may shine in one pillar over the others, but not in another" and that the opposite is "classes [are] equally able to contribute the same in all three pillars". In fact I reject the assertion that those are even opposites. So, again, I have no idea what you're actually asking because it's going to be different for everybody who reads the question.
I also reject the hidden claim that later editions don't have "niche protection" even using your flawed definition. The major difference in class design between AD&D and 5e D&D is the number of classes. The classes didn't step on each other's toes because there were fewer toes to step on. AD&D had Fighter (Paladin, Ranger), Magic-User (Illusionist), Thief (Assassin), Cleric (Druid), and Monk. (No, Bard doesn't count; Assassin barely counts.) 10 "classes," but they're actually presented as *5* distinct classes with some modified/upgraded versions that you might be able to play if you roll well. 5e has Bard, Barbarian, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Sorcerer, Wizard, Warlock. 12 distinct classes, all with slightly different mechanics, all with multiple archetypes or sub classes, many being classes formed from what are essentially the old subclasses. However, there's only maybe 3 to 5 roles in combat and out of combat. So obviously 5e allows multiple classes to fulfill the same role! There aren't enough roles for 12 different classes to each get a different role. That doesn't mean that every class is equally capable of filling every role.
I take a vague opening about niche protection to mean i should in my response clarify where appropriate what i am defining it as as opposed to some unanswerable mystery of the cosmos.And yet, 50+ people so far thought they could understand it well enough to answer as well as many others who have commented. I also find it pretty funny that you're basically saying "I don't understand what you're asking, but I disagree with it." If it doesn't make sense and you don't understand it, then how could you disagree with it? Can't have it both ways.

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