Right, this seems like the simplest solution. Would making saltpeter exceedingly rare possibly have some other impact on the world?

There were military air rifles in the 1700s. They even had some advantages over the gunpowder weapons of the time.

@TKK That's entirely true, and there's even a question on WB.SE about air rifles. The accepted answer, here (disclosure: it's my answer), points out those benefits. But there's also some big downsides that kept them from ever becoming widespread. Battlefield weaponry needs to be robust, and the Girandoni had too many fragile parts to be reliable; one tiny hole in the air reservoir made it useless. Also, the technology of the time couldn't mass-produce parts with the necessary precision.

Saltpeter also precipitates out compost heaps, depending on the type of plant matter used.

You would want the world to lack bats, as guano is one of the best/easiest sources of saltpetre. It may be that the lack of surface deposits and producers led to a lack of discovery of the reaction.

I need to reiterate my comment - we need to have chemistry very underdeveloped if want to have no propellants by 1900. Even if gunpowder is not available, many alternative should have been known by 1900, even if no one specifically searched for them.

+1 for the air rifles. BTW There is an anime heavily focused on military estrategies with magic (limited) and air rifles & air guns (alongside bows, swords & crossbows too) named: Nejimaki Seirei Senki: Tenkyou no Alderamin. Maybe is useful for reference.

You need to read the story of the saltpeter-men. Empowered by Parliament to gather saltpeter with a warrant to do so, they could forcefully enter your house and dig up your floor (if you didn't pay the appropriate bribes).

@TKK Yes but that's a technology based on existing firearms, with no existing firearms that technology may still exist but I doubt it, easier to build a better crossbow.

@NomadMaker I know many of the varied ways Saltpetre has been and is extracted and produced, I'm not saying you couldn't get it I'm saying if you don't know you need it you aren't looking for ways to get it, it won't keep gun powder out of the world forever or keep people from eventually stumbling on alternatives but it will slow the whole process down.

@TravisKindred You can have bats, just make them live deeper into cave systems, and or have them carry even nastier bacteria and parasites than they already do. That way guano hunting becomes a death sentence. You first need to be looking for saltpetre too, without surface deposits as a guide and original source there's no reason to go looking.

@Ash Good points!

During the Renaissance, saltpeter was typically extracted from manure. A lack of naturally-occurring saltpeter deposits is not going to stop the development of gunpowder, since once people start processing manure for use as fertilizer, discovery of its chemical utility is inevitable.

@Catgut Maybe they will maybe they won't, black powder was an accidental discovery the first time. Also why would anyone process manure for fertiliser? Just use it as fertiliser, it works better.

@Ash Composting was common practice in medieval Europe, mixing manure with unusable vegetation and animal products to form fertilizer. Once you start handling large amounts of nitrates, sooner or later it will combust. Such a chemically simple discovery, from substances in common use around the world, going unnoticed for nearly a millennium, is IMO completely implausible, accidental or not.

@Catgut And in a situation where you're handling composted organic material to boost your subsistence farm's yield combustion is a disaster you want to avoid not a scientifically significant discovery.

@Ash - unless this is a nitrogen-poor world, there will be many other sources of it, as others have pointed out.

@Alexander Sure there will, now can you tell me what a saltpetre source looks like if you don't know what saltpetre looks like? At no point and in no way am I suggesting that there won't be an eventual discovery in this field, just that this is the most effective way to stall development.

@Ash if we are talking about 1900s chemistry, any and all sources of nitrogen compounds will be discovered and evaluated. We can delay discovery of gunpowder by a few centuries, but if we hold back chemistry by some 50 years, 1900 won't look like 1900.

@Alexander Okay I think we're missing a step in pathways here, I'm assuming that without the role explosives came to play in war you lose a primary incentive that drove a lot of the systematisation and precision that played a key role not only in further developments but in the approach that allowed those developments to be understood, to my knowledge gunpowder was the first compound produced on a large scale using precise measures rather than "rule of thumb". As such without gun powder you don't get anything that looks like what we think of as 1900s chemistry for quite some time.

@Ash I'd say we will have a "steampunk 1900" with a lot of technologies (like petrochemistry) underdeveloped. Firearms would be novel and unreliable, but it would be difficult to handwave them completely away. By the way, dropping back to 1850 can solve this problem.

@Alexander Really, I'd have thought you'd need to go back to like 1500s to not have really major knock on effects be evident.

@Ash, hmm, you are probably right. I think 1850 is the latest when there is a plausible way to avoid having explosives and propellants.