I want to try Roleplay again, and since I can't use my Discord I'll have to see how DMs work for RP, mainly an In Character/Out of Character separation, a place to keep character info, and a possible dice roller.
You can make separate PMs for in-character and out of character interactions.
There is a section Extra Pages that you can use for posting threads about lore, for example (note that those threads will be public).
Also you can use Private Workshop to store any information you need. Those threads can only be seen by you and forums Staff.