Not one subreddit but a jumping-off search across the many design-focused communities on Reddit.
Reddit's anonymity tends to produce blunter, less performative feedback than LinkedIn-style networks: useful specifically when you want an honest gut-check rather than polite encouragement.
Best for designers comfortable with a more candid, sometimes brusque tone: if you want a supportive space instead, a group like Designers Guild is a better fit.
Quality varies hugely subreddit to subreddit: check r/UI_Design, r/userexperience, and r/web_design individually rather than treating "Reddit design" as one uniform community.
Explore various design communities on Reddit.
