Laurefindel
Legend
I think it's mostly how we describe or what constitutes a campaign that changed. Our AD&D 2E games were also a loose progression of smaller adventures, some with modules, some without, with characters (or series of characters) that would last for years upon years without any obvious conclusion. That, was the campaign.One thing I've found, which may just be the groups I've played with...
But when I started in 2E games, they were less campaign oriented. Not that it didn't happen, but it was more like an episodic TV show where you'd play a module, (or play through a dungeon crawl written by the DM) and then just move onto whatever came out (or was written) next. There was no real over-arcing plot.
These days the groups I've played with, there is an emphasis on the campaign from the start being more cohesive and planned.
Perhaps this is due to the modern trend of full campaign adventure paths vs shorter modules from the 2E era?
I can't really speak to how this evolved over time as I stopped playing pretty early on in 3E and didn't come back to 5E. It's more a direct comparison between now and then.
Nowadays, the campaign I put-up are much shorter in scope, much more focused, much more thematic, typically with a clear conclusion in mind. These campaigns last a year or two before we finish it and pass onto a new one.
But semantics aside I agree; the structure of a D&D campaign has changed significantly over the years.