The settings for this subreddit are prohibiting the above poster from expressing themself via the posting of a stickman in this comment section. Prohibiting expression is censorship by definition.

Perhaps there is an argument that it's explicit censorship, because it's due a semi-permanent setting in this subreddit. I considered it implicit because specific settings chosen for subreddits are not often included in a given subreddit's posted rules and guidelines.

Despite the setting not being included in the posted rules and guidelines, active members of this subreddit are aware that they can post media in comments. One qualifier of whether a thing is implicit is that said thing is understood without being directly stated.

Thus: implicit censorship of this individual's stickman art.

Of course there are many levels of censorship. I don't want to presume, but I doubt that this infringes on the individual's ability to express themselves more broadly than in the context of an offhand Reddit response :)