This is being done because Projects v1 requires repo write access, a serious limitation for letting people use it. Projects v2 isn't a great option because it lacks event support. Labels are pretty stable in GitHub, so this switches to that.
Note this assumes we're fine renaming "decision: accepted" -> "proposal accepted", etc. There are two reasons for this:
1) To make it clear that this is a proposal-specific label, versus something like an issue for leads label.
2) Removing the colon because it was causing trouble with yaml syntax.
This also adds the "proposal draft" label, mainly to complete the taxonomy.
I was considering whether this should be a proposal itself, but it feels like maybe it's not necessary because it's a fairly low-key infrastructure change, and I'm not sure how much people were relying on the project board anyways.
I tested this in a personal repo, basically just poking at https://github.com/jonmeow/test/pull/2
Jon Ross-Perkins