You can see this discussion about this proposition :
The new forum posting in Moodle 3.7 allows to directly type text in place, and one need to click on "Advanced" button to have the complete editor, allowing to use formatting (bold...), lists, links...
Some people nearly always use these things, and would better always have the complete editor shown when posting.
So there should be an option in user forum preferences, to define if by default the simple text should be shown, or the complete editor.
Perhaps there should also be an option for the admin to define the default mode, and allow (or not) users to choose it.