I'm working on a low-magic campaign setting right now, and have been having a lot of the same discussions going on in this thread. In the world I'm building, magic was more plentiful at one time, but due to
plot reasons the amount of magic available has lessened - like an aquifer that's been drained.
AiME has been a great resource for a no-magic campaign, but I've also found a lot of interesting stuff in the way the Witcher RPG handles casters; instead of limiting outright the number of spells they can cast at a time, spellcasters in the Witcher have to worry about drawing on more power than they can physically handle. Draw too much, cast too many spells, and they start taking damage and making save throws. It's an idea I'm still playing around with, but it's something I'm considering very heavily. It would of course have to be adapted, since the Witcher RPG uses a d12 system.
I haven't solidified everything I'm going to do for the setting yet, so take all of these changes with the requisite grains of salt. These are all just things I've been considering using in the final setting. Some of this is mechanics, but I think a good portion of it can also be achieved with how NPCs react to magic and how many resources are available for casters.
- Limiting the number of spellcasting classes; right now I'm working with bard, cleric, druid, and wizard. This has more to do with my personal class preference than any mechanical advantages.
- Remove magic from barbarians, rangers, and rogues. UA has a good resource for non-magical rangers.
- Move paladin into a subclass of cleric, where players can choose to be a sword-and-board cleric with limited spellcasting, or play a more stereotypical healer with fewer martial abilities.
- Halve the number of spell slots (rounded down) available to all casters, limit 1 per spell level
- To keep casters from being too squishy, allow for a bit of martial proficiencies. I'm thinking Lythande of the Blue Star in this model, someone whose primary focus is magic, but who's also no slouch when it comes to fighting.
- Require spell components instead of spell focus, and put a cap on how many uses a component pouch has. Right now I'm allowing 10 uses, but also allowing for casters to forage for easily replaceable components.
- Most magic items are simply not available, and certainly can't be found for sale.
- I'm also considering using the 5e DMG's rules for gritty healing, where a short rest is a night's rest and a long rest is a week. I also like AiME's approach to this in their journey mechanic.
- On the worldbuilding side, I'm keeping magic in the world very limited. Most NPCs will never have seen magic in their lives, much less anything on the scale of what the PCs may eventually be able to wield. As a result, most NPCs will react with disbelief, suspicion, or fear if the PCs use magic in an overt way around them. Magic schools are tiny things, perhaps one low-level wizard to a handful of students, or a druid circle composed of four people. Magic is available - at a significant cost - to the very wealthy, but even that isn't what it used to be. Old records speak of great deeds of magic done thousands of years ago, but in much the same way we treat that as mythology in our world, so do most people in this world. Magical healing at temples just isn't available most of time - much less raising the dead, and even basic healing potions are something you'd only find in large cities at fancy-shmancy boutiques for the ultra-rich.
- I really like the suggestion @Ath-kethin made about allowing the PCs to brew Keoghtom's Ointment instead of healing potions, and I'm definitely taking notes on that.
I'm aware that this could probably be more easily done using a different RPG system, like the Witcher, or Savage Lands, etc etc. D&D 5e is just what I started with and what I'm most familiar with. My goal is actually to build something that can be easily adapted for other RPG systems, but that's a personal thing.