The endless arguments across Meta show that there are problems no matter what policy is used. These problems are caused because questions that should have system tags do not have them.
We can maintain the policy and people will go against it when they add a screamingly obvious system tag and are told not to do so, resulting in irritation and upset for users.
We can change the policy and people will go against it when system tags are incorrectly edited in, resulting in irritation and upset for users.
(I could support further why each possible policy results in issues but if the countless Metas and this post are not evidence enough I really don't know what could be)
Reduce the number of questions that should have system tags but don't
I think something being ignored is the fact that a policy is only needed if there are questions which should have system tags and yet the tags are missing. This is why I believe that what we should actually be doing, instead of arguing circles about a policy and whether we should re-re-re-re-revisit said policy, is to better convey the importance of a system tag.
We should make it clearer to new askers that mentioning the exact system is going to be a requirement for having their question answered.
[different quote] We tell new users to take the tour, yet the tour doesn't mention the current policy about explicit system specification. This should be changed, or else we're going to keep running into this problem with newer users.
There is already some discussion of this: "Can our tag-prompt nudge toward including system?", but unfortunately, for the past while, we could not do anything about the situation. But given the recent change to the Ask A Question page, I believe this is an avenue once again worth pursuing.
What I would propose is some sort of change to the Ask A Question page so that users are more directly aware of the importance of system tags, or are nudged, in some way, to include a system tag, if it is applicable to their question.
Finally, and I cannot stress this enough, not every question needs a system tag; there are plenty of questions throughout the stack that are on player dynamics, gm-techniques, and other social (or at least non-mechanical) matters and these should not be required to have a system tag. If the Ask A Question page emphasized system tags is should be sure to note that such tags are helpful if (and only if) the question is directly about or deeply rooted in a certain TTRPG.