Well hey, I'd like to jump in on this if you don't mind.
Warlocks were introduced into 3.5 in the book Complete Arcane. All of the Complete books introduced a few base classes in their specific variety of character so if you're thinking of a 3.5 base class that's kinda like one of the core classes but a little different, it's probably from Complete Something. They're actually pretty close to their 5e version and I feel like their eldritch blast was the framework for how offensive cantrips in 5e were designed. The main difference is that the 3.5 warlock only uses Invocations and cannot cast proper spells, whereas the 5e warlock does cast spells and uses Invocations as an additional customization. The main reason I know this so deeply is because my old GM's big brother was kind of a power-gamer type and he frequently played the warlock due to eldritch blast and Invocations basically giving him a frequent source of powers and abilities that never actively ran out so he could always be the star of the show. So I've got a small brain poison that leaves me sour on them but they're still pretty cool even if the art for the "default" warlock in the book looks like a total dweeb.
Pictured: A total dweeb who insists he's very cool.
Anyway, that aside, yes, the difference between a wizard, sorcerer, and a warlock when it comes to generic fantasyposting is really more based on a vibe than anything else rather than any sort of specific mechanics like DnD puts on them in order to justify why you're buying another book for a new class when you already have a perfectly iconic set of classes in the base book. 3.5 was awful about that. Fantasyposting is fun because it hits upon iconic imagery and narratives that we're all familiar with and being pedantic about what class abilities you should actually have based on what synonym for "magic person" you happened to say is peak Internet pedantry that grates on my nerves to no end.
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