In every Berne Convention signatory country (which is pretty much any country that will matter to most people), published works are required by the convention to be copyrighted by default, whether or not there is a copyright notice, and the lack of a license means you have to assume you have none unless something else tells you differently. That basically makes embedding the license text pointless.

It certainly was common, though, which is/was probably because the US did not join until 1989, and prior to that US copyright law required a mandatory copyright notice, so a lot of people will have at least worked on projects old enough for this policy to have mattered.