This, A Player's Guide to Dunromin (there's also a GM's Guide which I will review later) by Simon Miles is one of those exceptions. Apparently it dates back to 1986, which is perhaps not old, old school, but certainly still in the late heyday of AD&D. Dunromin is actually the name of the main city of the setting, the world itself is named Barnaynia. If that name sounds funny, there's a reason for it, it's a presumably comical British pronounciation of banana, which is the shape of the world.
The history section is surprisingly dense, but basically the city was founded by a group of adventurers and is sort of a mix of Roman, Celtic, and Viking human races, including temples to those pantheons (plus Babylonian). There's also some elves and traditional fantasy races. It goes back a couple hundred years and a decent amount of stuff happens, some of which may be relevant to PCs (there's a civil war brewing) and periodic monster invasions, including one that is seemingly done by a Mind Flayer replacement called the Deep Race. They lack the face tentacles, and are more reptilians than Cthulhuian, but drain mental energy and are creepy looking.
There are a lot of new rules, about 100 pages of them. Some of these are basically just things from AD&D 1E/OSRIC, but tweaked slightly, plus new rules for things like critical hits, hit locations, and improving ability scores. There's also "Life Skills", basically non-weapon proficiencies.
Of most interest to me are the new classes. The first is the "Wild Domains Barbarian", which is basically a toned down barbarian, more on par with a fighter than the UA Barbarian. The most interesting thing is it has very expanded and detailed "frenzy" rules, which basically offers the character many different types of frenzy. For instance, giving a +2 to hit bonus or doubling number of attacks, running faster (and improving AC slightly), adding a bite attack, and more. There's even rules for Slaine comic "Warp Spasms"
The Diabolist, despite the name, summons more than just devils, and can bind outsiders into items to make short lived magic items. They also get a mix of cleric and magic-user spells, though mostly limited and dealing with summoning or detection.
The Witch isn't really a full class, but something of a split or prestige class that can also be taken and is completely separate from the character's other class(es). Their spells only go to 4th level, but are a mix of magic-user and cleric, including healing spells.
Ultimately, Dunromin is somewhat on the bland side, but does offer a good look at a 1e campaign setting developed from actual play. I find that interesting on its own merits, not simply as a place to use in my own game (which honestly, I won't). I also liked the additional rules, as I always welcome new classes and will probably use the Diabolist. I consider it a well spent $9.99