Simple and sweet:
Heroes numquam oblitterabuntur.
If I had to guess, I would say the idea behind the claim is that oblivisci (to forget) is a deponent verb and has no (semantically) passive forms, so naively, if one wanted to say “to be forgotten” in Latin and only knew that verb, one would be a bit stumped. Oblitterare does not mean “forget,” it means “to erase [from memory],” but in the passive it does not necessarily carry the idea of an erasing agent, and simply means “to become forgotten, to sink into oblivion.”
The argument is still a little silly, as there are probably languages out there that have no passive voice at all and are yet able to express the idea of everlasting memory somehow.