I disagree, I keep just about everything. Obviously the git is /the/ log for the code itself. But there's more to a project than code. I routinely take screenshots and just dump them into a "art dump" folder. It is nice to look back at ideas and features that went away, maybe for a "behind the scenes" look.
Git also doesn't keep ideas, brainstorm and discussion about them. I use Trello for this but it's just personal. But it lets you easily file things as "maybe some day" and "actually doing/done".