People bring that up all the time, but I can't really think of fainter "dark pattern" criticism than an optional email field. I bet nobody would even comment on it if it was actually required. Reddit should probably just go ahead and require it to eliminate the complaints.
Hell, I'm more ready to call it an easter egg than deception.
I'd wager you are correct that if it were required, nobody would comment on it except existing Reddit users familiar with their more privacy-respecting leanings from their past (ie. not new users).
The dark pattern is that there is zero indicator that it is actually optional and they appear to go out of their way to hide that fact. It is about being upfront with your intentions, and what you are offering users.
Hell, I'm more ready to call it an easter egg than deception.