How about the strange death of all ideological 'isms?

They may look good on paper but they all fail in the real world. Anything practical ends up being a mixture of ideologies --- some shade of gray rather than strictly black and white.

Thou shall not kill --- except.

I don't think libertarianism had a heyday in the 20th century. Sure, governments practiced more libertarian ideals in the 20th century than in the 21st, but that drop isn't near as drastic as it was between the 19th century and the 20th.

Its heyday was in the 18th century, when a bunch of British colonies diced it was better to break off into their own countries, then to let their leaders tax them and limit what goods they could produce. Those colonies formed their own governments, and agreed to federate an extremely limited subset of services that would allow free movement and trade between them.

Since then, the government practice of most libertarian ideals have been on a continuous decline.

It seems that at the highest level libertarianism has been co-opted with might makes right sensibilities. Which unsurprisingly correlated with "libertarians" who suddenly found themselves with a lot of disposable income...

Nothing strange about it. "Liberty" is a euphemism for unaccountability, and libertarianism, probed with steel, advocates only that business must be above the law. Most predictable ever.