To counter the above discord argument actually, there are at least three major factors as to why we can't necessarily rely on outside communication.
The first is due to the contract system. We might be able to handle our business dealings over third party messenger apps, but there's a great deal of value on the mechanical elements of in-game contracts concerning how they interfaces with players and npcs alike.
Secondly, there's the fact that information itself is a commodity. This has been mentioned in regards to The Weard and my example might not be perfect, but let's say you're knocked out and robbed by a bandit. If someone witnesses the crime, they're able to give that information to say a guard and perhaps get an investigation underway via bounty tokens.
And lastly, while you can certainly use discord to collaborate with the people you're working with, how do you reach the people you don't know? If you're running a business, settlement, duchy, etc, it will be necessarily to communicate with people you can reach over Discord or even the CoE forums. This problem is a bit unheard of now concerning the entirety of Elyria exists in these spaces, but that won't always be the case.
To sum it all up, forums and third party chat will absolutely be a part of the game, but SBS is doing a great job in making sure the communication within the world is integral to game play.