Disagree. KISS is for bigger things like architecture. Exposing an enum instead of a simple bool is a good idea that will save you time later. The only time to not do this is if you're exposing internal info, i.e., breaking encapsulation.
KISS, YAGNI, and then actually analyze your requirements to understand what the mature schema looks like. A boolean is the simplest thing that can possibly work, usually. Do that first and see how your requirements evolve, then build the database schema that reflects your actual requirements.
Although it always depends on what exactly you're really doing.