Never expected to learn something like this here! Very interesting, is the WHO involved in stuff like this, I assume so, right? Was China doing this somewhat secretly and we just, or recently, learned about it?
Colistin is actually a pretty old antibiotic, one of the earlier ones. It has some iffy side effects and is somewhat toxic though, so as soon as less harmful antibiotics were developed human medicine stopped using it. But animal medicine? Who cares that a few cows soon for slaughter have upset stomachs? As far as wiping out bacteria though it has been solid and reliable.
It was used in quite a few places for agriculture, including the EU, but it was stopped when human medicine basically asked for their drug back after so many other antibiotics became redundant.
Colistin is a drug of last resort (DoLR) so is one of those that is normally held in reserve for when all other antibiotics fail, and then it is normally administered under supervision to ensure that all relevant nasties are dead before the patient is turned loose on the world. There are a few others, but all of them (I think) have managed to fail against some bacteria at some point. It could be that there are other antibiotics that they're not telling anyone about at all which are super super top-secret. But bacteria is a really tough customer. Viruses (ebola, AIDS, rabies, influenza, smallpox etc) get all the glory but bacteria is the real star in my opinion. Among other things viruses are way easier to provide viable vaccines for.
As for colistin, China dragged their heels for years despite international pressure, and only banned it in October of this year, and it looks like the genie may be out of the bottle on this one. Bacteria can share genes even cross-species in a weird, borg like mechanism. This is an oversimplification, but imagine seeing, for example, the fangs of a tiger and thinking "that'd be useful!", taking the tigers "fang genes", and putting them in your own mouth. Bacteria can kind of do just that. So even if completely harmless bacteria become immune to an antibiotic that can have terrible consequences down the line, and bacteria immune to colistin is cropping up in more and more places.