Writing design documents and e-mails is part of development process too. And chatting on Slack is part of that process. It doesn’t matter how you define development process here, it gets us nowhere.
Only important thing is that we have tens or hundreds project management tools available. Many of them are free of charge, like Trello. We don’t this kind of tool in engine - because they already exist.
We need tens of other features implemented in engine, rendering, gameplay systems, etc. Adding PM tool would be waste of time here.
You don’t need UE4 tool to plan properly. You need any tool to add, assign and manage you tasks