Keynes had micro and macro economic theories. While his micro economic theories/models were quite good and still work, his macro-theory (the model of models) is what most people who say they're Keynesians tend to be talking about. Those never worked. Hayek won the nobel prize in economics for showing why in the 1940's (Dispersed Knowledge). But Keynes' macro-economic theories broke completely in the 1970's with Stagflation: a concept which couldn't exist according to Keynes's original models.
Well, if you're proven wrong, but have a career being a specialist based on those wrong theories, what do you do? Rather than admitting their prophet was wrong, Keynesians just reworked the models (to allow an exception for stagflation) and called those new theories/models, "New Keynesianism". Their religion of Keynesianism was was too important to face contradicting reality, so they forked history. Thus pre-1970s and post 1980s Keynesianism is based on different models... but has the same flawed beliefs. We know that the new models have also never been accurate in any major prediction of trajectory in a recession or a recovery, so while they're not as glaringly wrong, the models still don't fit reality or actually work.