People have been treating Cthulu as open source for years, and nobody is making any moves.In the E.U., yes, as of 2008. In the U.S., not so clear. However:
a) It sounds like it's also unclear who would have the right to make an infringement claim on use of Cthulhu, and
b) All 1926 works (to include "The Call of Cthulhu") will be entering the public domain in the U.S. on January 1, 2022, so in five months the question will be moot.