So you agree with the OP that 5e is not the Golden Age of lore generation?
Honestly... We've NEVER had a "Golden Age" of Lore Generation and I think the core conceit of the concept is flawed at best.
Early D&D had settings that were pretty basic "Slightly different from Lord of the Rings" material that was just used as a backdrop to random dungeon crawls and lootgoblinning. Murder Hoboing was the name of the game and the story along the way was a special treat for killing the -right- people rather than -everyone-.
People complain to this day about "Railroading" because they want their absolutely freeform no central narrative game where the story is character-driven but also utterly unplanned. Which, y'know, can be cool and all, but doesn't jive with what writers can provide when they're creating adventures for mass-marketing. The story is background in that sense.
Things evolved. Settings became deeper and more complicated. Sometimes to an... offputting degree.
Don't get me wrong. There was a -lot- of lore produced for FR and other settings... But there's a reason I say it's pick and choose. 'Cause some of it was just -bad-.
In the giant 2e GLUT of material for settings they overdid everything. So much material it was sitting on shelves in TSR warehouses never to be sold 'cause they couldn't move the previously written stuff.
Does that mean 2e was bad for lore? Ehhh... It certainly had a lot of it. And a bunch of it was REALLY GOOD. But just as much if not more was Surf Druids and Dark Elf Mating Rituals.
Then came 3e... hooo boy. 3e.
3e had some decent lore-support for Forgotten Realms... Actually cool lore support for Eberron. And then one-off "Setting" books which were largely lacking. Ghostwalk, for example, needed a lot more support than it got. But rather than setting glut we got Crunch Glut. The OGL allowed WotC to outsource most of the setting design stuff, even for books like Ravenloft, which lead to weird "Is this canon or not?" questions.
And then 4e streamlined crunch and provided a fair number of campaign setting materials. Mostly for Nentir Vale and FR (because FR is always there), but, y'know. Most of what it produced was updates and retcons. Splashing a whole continent of Dragonborn into FR just 'cause you want to add a new PC Race was just annoying. And then they got rid of it in 5e 'cause they realized it wasn't actually needed.
And now 5e. Which has done the most revising and the least writing of settings so far. Rather than produce tome after tome of material for 5e they provide one or two books which provide tone and identity and let you pick and choose what to put in it from a vast back catalogue of materials that were once Canonical in their entirety. Ravenloft is perhaps the best example of this sort of update in that it basically has a Multiverse of Dread Realms slowly sliding into oblivion and each one is just as "Real" as any other so you can combine the modern ones with the 2e ones with the 3e 3rd party ones and they all work just fine. You can even ignore Lord Soth's appearance-absence-dual existence nonsense.
So... to sum up: D&D has never -had- a "Golden Age" of lore. It's had times where there was a lot, a little, very focused, very diffuse, and now referential lore that directs you to previous materials and makes room for what you want. Does that make it a Golden Age? Not really. It just makes it a 'Self Aware' age. Does having a massive quantity of lore for various settings and worlds make a Golden Age? Ostensibly it could if the significant majority of that material was of high quality. Does having a very focused growth of material only for one or two specific settings make a Golden Age? It certainly could if your setting is the one they're focusing on, sure.
But there's never been a true Golden Age of Lore for D&D. The premise itself is flawed and is going to be based almost entirely on personal opinion in a vast sea of personal opinions that either obliquely line up or find themselves at cross points.
1e had the first
2e had the most, for better and worse
3e had the most focused
4e had the most retcons (seriously, the whole new casting system required massive rewrites or retcons of three of the chosen settings. FR, DS, and Eberron!)
5e has the most accommodation of conflicting materials from previous editions
Which one is the best? Which one is the Golden Age? None. Or all. Or the one you, particularly, like best. Or the one that fits your personal criteria. But there's no one that satisfies all. And can't be. S'why 5e went for accommodating any previous edition as much as possible.
