We usually promote only on discord the forums, and the icon that you click on to join the server after you click multiplayer. Players do use withers to promote their own sales or whatnot in their town so I could see this happening, but some people might not want the clutter on their screen.
I will agree with Desu here, in addition to that advertising this via boss bars seems a little unnecessary. Those who would like to support the server will either be in the discord already and/or checks the shop often. In addition, the website usually gives a message when such a sale occurs.