OPEN-ENDED PUZZLES ARE NO LONGER IN SCOPE. Used when no answer can be guaranteed to be the last, best one. If a puzzle instead has a provably optimal solution, tag with [optimization].
Per a community discussion, truly open-ended questions, defined as those which expect multiple answers and with none that can be proven as optimal, are no longer accepted on this site.
There are some open-ended questions that are still allowed. These include optimization puzzles where there is a provably correct answer (which the poster ideally will know ahead of time), and questions asking about a specific aspect of puzzle-creation. For such questions, use these tags (or other relevant tags); do not use this tag.