So whenever I GM an RP that uses a discord I have a specific rule when it comes to players planning and discussing stuff etc: 

'Nothing is official until it gets posted in the OOC.' 

Basically I don't think its fair to make players have to trawl through hundreds of discord messages to find the information that they need to play the game or roleplay with other characters. I've seen people decide stuff between them in discord, which they then haven't added to the World Compendium or their character sheet, and then gotten mad at people when they missed said details or got them wrong in an IC post. I also think discord can have a nasty tendency of making the people who just happen to be online at a particular time becoming the driving force in decision making that affects the whole party/roleplay.  

Discord is good for chatting, spit balling, hanging out, and quickfire questions. But if its something important that people will need to know in character, or if it's game critical decisions and announcements, everyone should have a chance to be able to see and respond to those, not just whoever gets there first and monopolises discussion. Hence, my rule, nothing is official until it gets posted in the OOC.