Use Steam to advertise a competitors platform.
Back around 2013-14 a Steam group called Hookups would advertise deals for games and would include deals on Origin. Then one day the group completely vanished without warning. It took a couple of months to find out the group was banned for advertising a competitors platform. There is no problem advertising a store that sells Steam keys as they still get money, advertising and customers by supplying keys. Advertising for a direct competitor though, is generally considered a bad thing and if you try it in a real world store you would quickly find yourself in serious trouble.
Epic Games Store is a direct competitor and while the people running Steam are generally too busy to notice what happens on Salenauts GG.deals Steam group page, if they do notice the Group page will be completely shut down. I highly recommend using Steam announcements on group page only for Steam games, and removing any existing announcements. Hookups got caught by someone reporting on different matter but all it takes is for someone running Steam to become aware.