Star Trek (ToS) was not "communist".

 As one who has studied and written fairly extensively about communism specifically as well as ideology in general:
Star Trek (ToS) is no variety of "communist".

You might think otherwise due to a bad definition of "communism" (there are several).

Star Trek does portray a militarized culture. Militarized cultures have elements of fascism and communism in them, which is why very often the show is facing off with either ideology. It's socially progressive liberalism: no indication that property has been abolished, there is clearly the norm of tolerance: how to even govern an earthly federation let alone an interstellar one without tolerance? But we defnitely don't see anyone railing on about class war, collective production, and the closest to a "five year plan" is the "five year mission" of space exploration. There's no outlawing religion, no restrictions on free speech, it's depicts a liberal society depicted, but a militarized one.

It's basicaly the U.S. Navy in space.