Promote editing down over deleting, discourage deleting, but make it clear it's allowed
First of all, deleting answers in the Sandbox doesn't save any space for 10k+ users if the answers aren't also edited down. So, the first thing that should be done to a posted proposal is that it be edited down into a stub, and I encourage people to edit such proposals down if the author doesn't do so in a reasonable amount of time.
Additionally, deleting answers in the Sandbox, while saving space for users with less than 10k reputation has a number of problems, as pointed out by Wheat Wizardpointed out by Wheat Wizard. Namely,
- Feedback and editing histories of the question are hidden for a number of users
- It makes it a lot harder to search for duplicate/failed challenge ideas, as deleted answers are almost impossible to search for.
Therefore, I suggest we make it clear that people should definitely edit their answers down, and, while it isn't recommended, they are welcome to delete their posted proposals