No big mystery. And no need to be coy.

I didn't bring it up simply because it isn't germane to survival other than to note you don't want to mix chemicals much more reactive than water with potassium permanganate.

Potassium permanganate is a component in a variety of pyrotechnic and potentially explosive formulas. None of which have much practical value in a practical and real-life, as opposed to militarized fantasy, survival situations. It's potential for such uses, which isn't really all that great compared to other common chemicals, has meant that I don't see it as often as I used to. It's most common use presently is in water treatment, removing iron. Goes for about $25 at a hardware store for what I think is a five pound, maybe ten pound, plastic jug. I got mine from a chemical supply house in a one kilo bottle for $4. After twenty years I have better than half left.

The general rule is anything strong enough to do a serious job is strong enough to harm you if handled incorrectly. Used with care it has legitimate and effective survival uses, I have used it in a variety of ways, but you need to be careful. It is always helpful to try these sorts of things out at home in a non-survival situation and gain experience. A lot depends on what sort of things you like and what works for you. Potassium permanganate is not survival 101, safe for casual users, or children.

In comparison chlorine dioxide, a favored chemical treatment, is a fine water treatment that effectively handles most biological hazards but you need to buy it fresh and either use it or discard it. Shelf life is widely listed as one year but many packages are not clearly marked with manufacturing date or expiration. But even one year is optimistic if you store it in the trunk of your car. Or where heat is a potential issue. If the water is really hazardous your better off making sure the chlorine dioxide is six months or less old.

Old school iodine tablets store for years but exactly how long depends on who made them. Potassium permanganate can store for years. Iodine crystals have essentially unlimited shelf life. Which makes them a good candidate for remote sites, caches, bomb shelters.

Some people seem to handle preparation as a hobby. They are constantly reworking, tweaking and updating their gear. Replacing expired chemicals might not be an issue for them. My preference is to set up a kit and check it once a year. Once every two years is better still.