The SIL and Seed Company base plans have several tasks that are not done in Paratext, such as Community review. The intent is that if a task is very important to a translation project then it can be made part of the overall plan and be tracked.
By the way, other than the automated checks, Paratext does not verify that you have actually completed any tasks that you have marked complete. It is very much a self-reporting system.
For Translation Briefs in particular I think various entities will come up with their own forms or templates. I believe that The Seed Company does have such a document. As was said in another comment you could put that document in the shared folder if it is not so large as to strain the bandwidth available for Send and Receive.
If you think that the brief should be part of the project as an appendix like the glossary or something, then maybe you could make the suggestion to SIL International Translation Department who have the responsibility for maintaining the plan.