Currently the Don't Ask page of SE sites indicates "You should only ask practical, answerable questions based on actual problems that you face" however Good Subjective, Bad Subjective, as well as a great many non-technical Stack Exchange sites delve in to topics which aren't always problem based.
Yes, good subjective is hard to explain and harder to understand at first glance, but this wording would be greatly limiting against good subjective questions if directly followed. Additionally, it technically means that canonical questions that expert users post themselves in order to answer for the benefit of others who they could foresee having a well understood problem are also technically bad practice.
Both of these scenarios are already common and beneficial practice across the Stack Exchange network, but the outdated wording on the Don't Ask page is confusing to less frequent users who are familiar with the documentation, but not with the day to day way that the network actually runs.
I'm not 100% sure what a better wording would be that makes it clear that there are situations where a good question doesn't have to be about a problem you face, or even a problem at all, but also not lose that if a question IS about a solvable problem that you face, it's probably much more likely to be a good fit.
Perhaps something along the lines of "Questions need to have answers that can be supported. Questions that aren't about practical problems that you face or specific knowledge that you seek are likely to be closed."
This wording reinforces that questions need to be answerable with answers that can provide support, makes clear that specific knowledge questions are ok and softens the language to make it clear that there are exceptions (such as canonical postings) where portions may not apply.