I just simply test the "wss://gateway.discord.gg/?encoding=json&v=9&compress=zlib-stream" from devtool's network tab to send message myself, so i found that the "binary message" is discord's message. it looks like using some library to compress data. (https://discord.com/blog/how-discord-reduced-websocket-traffic-by-40-percent)

(yeah i know it is not a perfect answer so i wanna write at a comment but site says i can't, sorry)