Does the term "class" imply an exclusive niche? Vote away.
Obviously it doesn't. Otherwise the voting would be meaningless. It's also incredible hard to describe what "class" means in RPGs. The only pertinant answer is the type of character you play. Classes are the primary answer, when asked: "What character do you play?"
That doesn't actually lead to niche protection, even if the class has exclusive abilities. Imagine two people playing the same class.
It follows that niche protection only works if the players cooperate in establishing it. Classes can help in this negotiation, even if they have no rules to tied them at all. Imagine a Shadowrun group. A player might say: "I'll be the Decker." You immediatley know what to expect, even though there are no formal classes in the game at all.
The archetypes in Shadowrun are in a way even more reliable than D&D's classes. That is because, if you do not know an archetype in Shadowrun fitting your character, you will probably try to communicate whatever your idea is. In D&D, fellow players might look at the character sheet and, having read the character class, do not even bother to ask.
But classes can serve other purposes, too. They can be starting points for imagining a future character. World of Darkness games are prime example. When you choose a clan or tribe or whatever, you can either play it straight or subvert it. Like you can play your Ventrue as ambitious upper class person or do the opposite. And community will not expect all Ventrue to by typical Ventrue. That would be boring.
The problem is that in D&D these too functions, helping in niche protection and establishing a baseline character to maybe deviate from, are conflated. WotC tried to adress this problem in 4e by establishing roles (Striker, Leader, Defender, Controller). That immediately changed the discourse in the community. Now, people were asking how to make good Leaders, Defenders etc. It didn't generally improve the discourse.
There is a game, though, that does a good job in establishing niche protection via classes:
Old School Hack. The rules are this:
[*] If you choose a class, no other player may do so. (Don't be a dick about it.)
[*] Each level you get a class ability.
[*] You may choose abilities from classes other than your own, but you can never have more cross-class abilities than abilities from your own.
[*] When a player plays the class that you want to take a cross-class ability from, you have get permission before taking the ability.
That actually works because it doesn't consider niche protection as fait accompli but as a form of negotiation. If the player who has a certain class ability on the list, but doesn't want it, and you think it's really necessary or fitting for your character, you can just ask.