That's the thing though - as long as something is banned, it doesn't stay forgotten. We see Tiananmen Square pop up all the time not because of its major historical significance, but because of the attempts by certain governments to censor it.
There are better ways to forget something collectively as a culture. Right now if you google "New Zealand shooting", half of the articles are about the banning of the information. The banning itself gave it more attention, and realistically it wont stop those who want to find it from finding it.
What I'd rather have seen was NZ focusing on the victims and the families. Drown out the manifesto with positivity. Focus the news cycle on the communities that have been affected; show them coming together and embracing each other. Make what is remembered not the shooter or their manifesto, but how the community responded.