It is very controversial. The grammar police sometimes give long speeches about this “incorrect” plural. The truth is that it didn’t spread because people thought it was correct. It spread because it is a great word! Because “octopuses” is boring! “Octopodes” is less boring than “octopuses” but both are still too boring.
According to Merriam Webster, it is not incorrect and there are actually 3 different plurals for the word octopus, including octopuses, octopi and octopods.