Just from personal experience. I have never had a decrease life issue with any CPU that I have overclocked.
Now maybe, granted, I didn't always have any one CPU, in any one system, for a exorbitant amount of years.
So, are those CPU's still operating today? I don't have a clue. So I have no reference of knowledge, that
there is a issue with lifespan or not. Other than maybe pushing CPU to there absolute, failure limit frequently?
Then of course The CPU life is going to suffer.