Well if you can see condensed water vapor in open air, it's most likely nucleating around something. And that something is probably the combustion products from the jet fuel. I don't know exactly what each of those thousands of molecular structures are, but burning fuels in general can produce a whole host of compounds including Polyaromatic Hydrocarbons (PAHs) that can be rather carcinogenic. And that pollution most likely settles somewhere on the planet surface eventually.
As takeda said in another comment, it might not be worse than more local sources like cars or trains or power plants, but it's very unlikely to just be water, and probably not healthy.
I'm not saying it's a deliberate spread of chemicals and it's certainly not the mind control nonsense that conspiracy lunatics think. No, most combustion products are just waste with no real use. But calling it "just water" or "harmless clouds" can't be entirely right unless you believe carbon atoms can just disappear.
We don't have a good alternative to planes so it's probably better to require the industry to offset its pollution. Not than anyone wants flights to be more expensive. But as usual the ultra-wealthy take the most flights yet don't pay as much as they should.