We played the Forgotten Realms in the early AD&D 1e days, way before the 2e novels caused havoc with author NPCs and ever-expanding "don't touch this author's area" world design, which honestly caused a strange fractal repetition in cultures and places, so every writer could baseline a "fantasy setting place" as their own. We never had invincible GMNPCs running around or organizations in every area that could step in and save the day.
It was just a world, and it was ours.
The 4e disaster destroyed the world, and for us, the world's history ends around the above-depicted 3e book. We hated the 4e shoehorn of eladrin and dragonkin in the FR lore, and this foretold today's homogeneity where "every world needs every race and background," even if it does not make any sense at all. What makes a world unique is who lives there and accepts the limitations of the culture and setting; otherwise, we will need Klingons and Vulcans in Star Wars. Along with anthropomorphic animal races, space goblins, dragons, and...
The tendency to put everything in every world becomes obsessive-compulsive after a while. It takes away anything unique and exciting about the setting and makes it just like every other place. This is the equivalent of chain restaurants and applying that logic to fantasy settings. You go to a place looking for some great local barbeque, and your friend from there takes you to Applebees.
Diversity of background and culture can exist in settings with a world with just humans; just look at Earth.
5E, as far as I know, still needs a Forgotten Realms Campaign Guide (if we ever get one). Wizards stopped making setting guides except for books linked to Magic: The Gathering and a few other oddities that have been underwhelming and feel like filler. The great campaign settings these days are written by 3rd parties (see also: The OGL Disaster of 2023), and if I want a fully fleshed-out "official" setting, I am buying older setting guides and adapting. In some ways, it is a good thing since they won't screw them up.
We Played Low Fantasy
When we played in the Forgotten Realms, it was a low-fantasy game. It was terrific; we were so used to the chests filled with a million gold pieces, PCs with gloating castles and armies of djinn and gold dragons, and characters with stacks of +5 magic items so deep they walked around with a chestful of gold bling Greyhawk 100th level characters - having an AD&D world that was down-to-earth and realistic felt so good.
There was even the concept in here (enforced by the gods of Dragonlance in their world Krynn) that if characters got too powerful, they were asked to leave the world and banished. And there were strict "transfer rules" in place for any visitors to the world; you could not be disallowed race and class combos, unrealistic ability scores would be trimmed down, powerful magic items disallowed, technology banned, and wealth for incoming "residents" severely restricted.
Yes, the Forgotten Realms was this strange European country with all sorts of limits on citizenship. And we were sure if a character ever got too powerful, they would not be allowed to stay; most likely, they would get sent to Greyhawk with the rest of the power gamers and 100th-level characters.
Monsters? What Monsters?
Many town maps had no walls and natural defenses, so we assumed these areas were already settled, and the land tamed. This was not your typical B/X world where you step ten feet into the woods out of town and start rolling random encounters with giant beetles and goblin war bands. There were militia and patrols in settled areas, and towns existed peacefully in the settled areas of the world. We did a lot of roleplaying, and this was one of our first "fantasy RP" worlds where the conflicts were story-driven and not "the monster of the week."
Evil wizards, bandits, and other humans were often the bad guys in these stories. You get towards the borderlands of each kingdom, and you start seeing orcs, gnolls, and other humanoids. The silly monsters of Greyhawk (gelatinous cubes, mimics, ropers, piercers, etc.) were not present since we saw those as a little childish and "dungeon-y" for a mature and realistic world.
It did feel a lot like Lord of the Rings, in fact. Elves stuck to their forest and were rarely seen outside of it. The dangerous lands on the borders were where the King's forces battled the armies of chaos. If you found "adventure" in a settled area, it was something special and mysterious, like a lost ruin nobody knew about for hundreds or thousands of years. Elves and dwarves stuck to their lands, and seeing visitors in town was a memorable and fun moment worthy of roleplaying the meeting.
Please Stock my Dungeon!
These days I get the feeling that campaign settings exist only for a reason to provide a map to throw fully-stocked dungeons on. We ran ours as a realistic world, devoid of powerful GMNPCs and some of the silly "dungeon ecology" stuff Greyhawk enshrined in the hobby. Whenever we talked about "the Realms was a cool place" with other players, we got moans about the omnipresent GMNPCs, and we were left scratching our heads - having skipped the novels and most of the adventures to do our own thing.
We didn't have the cartoon beholders and drow who ran thieves' guilds in Waterdeep or most of the godling and powerful monster stuff in the Baldur's Gate games. We skipped the first "time of trials" and felt it was stupid to cause an in-world event to happen to explain the 2e rules changes. TSR ended up destroying the world for us in the 2e transition, and we moved on to superhero games and sci-fi.
There is this tendency in D&D the game to over-magic the world into this silly high-fantasy superhero mode, the same one established in D&D 4e. The 5E game has that feeling in spades; everyone is a superhero starting out and gets more superheroes by the campaign's end. It is fun, but it is also very tiring and lacks challenge.
GURPS or Dungeon Fantasy
If I ever revisited the Realms, I would do it with GURPS or Dungeon Fantasy. I would want a low-fantasy, gritty, realistic game to match our original experience with the setting. I do not care about having "every monster in the book" and "every magic item on the list" at all. Our Realms never had them, and most of the conflicts were between humans and humanoids, and actual monsters were rare and unique. Magic was not too commonplace either, and wizards were special. Most battles were fought without magic, and magic was a relatively unique and rare power. Most classes did not have magic powers either, and you could get by in the world being a great ranger with excellent wilderness skills.
One evil mage with his network of assassins, spies, mercenaries, and an allied humanoid tribe? A great campaign villain. Monsters? They could be summoned in and were very rare and unique, or one powerful owlbear would be the "boss monster" of a dungeon. Dragons? Maybe you see one flying far overhead and wonder.
I thought Pathfinder 1e would be a good fit, but after I thought more on the subject, the entire 3E ecosystem and Pathfinder have way too much "stuff" to ever be of use in the Realms we played in. I would not use 95% of the classes, monsters, backgrounds, and items in the game - and it would be more of a chore cutting things out than recreating a world I knew.
Castles & Crusades could do it well, but it would still have higher magic than our game, and again, I would only use a small portion of the game. The GURPS combat system is a lot more gritty and realistic, and that would give a good "feel" to the game and enforce the deadly and brutal nature that would feel right for the experience I want. C&C is still a great game, the best AD&D feeling game out there.
Much of what ended up mattering in D&D and even Pathfinder was absent in our version of the Realms. The world was fantastic, like Lord of the Rings, and in its own reality bubble.
And it was a fantastic place while it lasted.