The only things that's true now is that there's more laxity around consolidation of power in big business. The core tenets of capitalism haven't changed in thousands of years.
Are you confusing capitalism with class society in general? Yes, class society exists since the neolithic revolution. Those economic systems had barely anything in common with capitalism though. Even medieval feudalism is very different.
And yes, late stage capitalism is a term. It was coined by Lenin in his book about Imperialism. You might not agree with the term but that doesn't mean it is not real.
I think it is very obvious that 20th and 21st century monopoly capitalism is qualitatively different to 18th and 19th century free market capitalism.
The effects and consequences of capitalism under feudalism or the age of slavery is, for example, fundamentally different from capitalism under a freer modern democracy. A slave or serf did not have the opportunities of capitalism, which changes how the system behaves and its effects.
The term "capitalism" becomes kind of meaningless, because it just describes a broad set of mechanisms. In the case of the question in this thread it is much more descriptive to include the context of which it exists within.