I found out about it because of a hacker news comment discussing the Reddit drama. I'm pretty upset that I didn't discover it on Reddit. I assume any mention of Lemmy was marked as spam or was I just not browsing the right communities?
I think it was on /r/selfhosted or the Awesome Self-Hosted repo on GitHub about a year back. I definitely don't think it was censored by mods/admins or anything on Reddit (back then anyway). It just seemed to lack the critical mass back then. In fact, I was looking at it as a way to host a private, non-federated link sharing board for friends but never got around to it.
Fastforward to this Reddit shitshow and hearing about lemmy.ml and lemmy.world and other bigger instances I finally decided to register and try it out!