Coming back to this issue, I believe we need a custom on-hold message for questions with a missing system tag. Two years later, it's one of our most common issues and perhaps the most common single reason why new questions are closed.
As I described in Can we add a "please tag your system" on-hold message?, a new user unfamiliar with the system tagging rules asked a question without stating their rules system, and it was put on hold. However, the user was given a generic on-hold message:
Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.
The user saw the on-hold message responded by giving more information, but didn't tag the system. Nothing in the prominent on-hold message or the How to Ask page asked the player which system they were using, and site rules forbid inferring it. A helpful user did give the generic welcome message and included a request for system tag, but this went unnoticed by the user, perhaps due a tendency to bury tag requests within a generic welcome message.
We have separate custom messages for closing off-topic questions about board/card games and video games, but I rarely see anyone make this mistake nowadays. In these situations, as I understand it, we could instead just move the question to the relevant RPG.SE site.
Considerably more common is new RPG players, unfamiliar with the tag requirements, asking questions (usually about the latest edition of D&D) only to receive discouraging close votes and downvotes. We should communicate the site's needs as quickly and effectively as possible to help these new users get their question re-opened.