Most have been true at some point. They all (most) have a reference to something that once made perfect sense.

For example: Pot calling the kettle black. Most kettles were black at one era in time. Now they can be different colors.

But here are some [more] modern ones:

'A 90s one: all that and a bag of chips' a slang saying originating in the nineties. however it can be classified as "true" since many people would get a free bag of chips with their meal and it was "sweet" or "cool" to get that hence someone who is all that and a bag of chips is "cool" or "sweet" in the sense that "sweet" is also a synonym for cool. The true origins being popularized by Public Enemy as a 'way to describe meals that black people used to eat' (Professor Griff

'The internet is dead' said when we get the nostalgic shock of an era no longer the golden age of internet. And it is true, many things that were great about old internet are now gone or modernized into a streamlined mess of paywalls and adblock-blockers. This one was explained to me at one point on YouTube.

They are called idioms in a sense because some of us can't help but feel uneducated when we cannot figure out what they mean or why that phrase would come to mean what it does. But it sure does make the past a bit more interesting. This last part is just a joke. I once heard a doctor explain what idiopathic meant to him as he stated "he feels like an idiot because he cannot find the out what the cause of something was" But Idiom means a phrase unique to language.

Edits: clarity and missing information.