Lea user can create projects

A logged in user can create a new project.
I can see discontinuing a category, to prevent new projects from being created in that category (say, if micro-hydro is banned, federally, or something), but retaining the existing projects within that category. I'm guessing I'll need to have some sort of "availability" tag, so only available categories will be visible in dropdowns, etc., and not allow categories to be deleted. Does that make sense? Is there a typical way to handle that kind of scenario?