Yekke2:
Fair question. (By the way I assume that when you wrote “the means to the end” you meant the opposite.)
If I may venture into what may very well be ??????? ?? ???:
Perhaps the quest for immortality is not inherently good or bad. It depends why you want to conquer death. Someone who wants to conquer death for the proper reasons (whatever they might be) would not so via horcruxes since it involves murder. Perforce, one who makes horcruxes is attempting to conquer death for the wrong reasons which is indeed something which should not be spoken of. Dumbledore, though he didn’t use horcruxes, knew himself that he was doing it for the wrong reasons, hence he was no better than Voldemort.
I realize this is a tremendously dochek answer and it doesn’t even really explain why a horcrux would be the “wickedest of magical inventions”. However, despite the fact that creating a horcrux would not be inherently any worse than any other ways of conquering death, it could still make sense that “we shall not speak nor give direction”, in that the assumption is that generally speaking, the creation of a horcrux is the only practical way to conquer death (i.e. anyone can do it), so by censoring he instruction manual, you can effectively prevent almost everyone from conquering death.