plebbit is a text-only protocol, media is embedded from regular links like imgur, not hidden links. Nobody can stop anyone from connecting to a subplebbit posting whatever in the form of text, because you connect p2p, and this is the same freedom you already get with other p2p protocols like bittorrent and bitcoin. This means you can post an illegal onion link in the form of text in a subplebbit, just like you can do it on bitcoin or on a torrent (but the liability belongs to Tor). These p2p protocols are free speech tools and they are unstoppable, eg. https://www.bittorrent.org/introduction.html