Im trying to reproduce the same behavior on my new Discourse instalation: when you start typing the topic title, once category is assigned, in meta.discourse.org you get suggest topics just in case you are about to publish something that has related content. Is this a plugin or option I have to activate?
It happens by default. If you site is brand new, it’s likely that you don’t have any topics that match, thus the “your topic seems similar to” warning will not appear.