Talking online about the R18+ debacle is one thing. Email is roughly on the same level.
Snail mail, though, is still taken seriously. It implies more thought and care has gone in to what you have to say than just quickly typing up a post on the internet and pressing send. That’s how society works at the moment.
As such, you should write snail mail to the Hon Brendan O’Connor MP, Minister for Home Affairs, and request that the discussion paper for an R18+ rating be released.
Again, the usual modus operandi when a website asks you to send a letter is to provide one that you can print out and send. I won’t be doing that. Rather, I encourage you to write out your own request and mail that. I can only see multiple separate responses going over better than a canned response sent 100 times.