I'm all for trying to find ways to cut down on e-waste, but I don't think this is it and it being water soluble could potentially shorten lifespan of devices because they could unintentionally disintegrate when it comes in contact with water.

This also doesn't really fix the problem if electronics end up in a landfill as there's still plastic, glass and batteries that'll sit there plus lead is still a problem if it does use lead solder.

At best, it only helps if a device is properly recycled and it doesn't really do much if it isn't. The best solution is still to not needlessly buy new devices and use what you have for a bit longer and companies should support their devices longer. I doubt the latter will happen though because pretty much every company will favor profits over the environment and they'll continue to greenwash to say how they're carbon neutral. Unless you go with something super cheap, most smartphones should easily last 5+ years and then not providing software or security updates after a couple of years is purely a business decision.