I would say it was widespread, but it's now fallen out of favor. Alexandrescu got the name wrong 25 years ago† and that got embedded into early boost, but by the time that blog post was written I feel like I'd mostly stopped seeing it being used by C++ experts.
† I don't know if the original mistake was his, but he certainly did a lot to spread it.
A simple Google search shows no shortage of recent websites and documents including fairly authoritative references using the term functor to describe a type that overloads operator ().
Heck even the author of this article, Raymond Chen, uses the term functor as recently as 2020 to describe such a construct:
† I don't know if the original mistake was his, but he certainly did a lot to spread it.