The blanket ban should apply to all systems because [designer-reasons] is equally unhelpful to all systems.
The extra moderation effort on designer reasons is a good reason to ban them on it's own, but the tag in general isn't useful to the site, and encourages question-asking behaviour that is specifically called out in the Help Center as bad.
I don't feel that there's any practical use in asking about the designer's intent directly in a question. Our overall goal here is to help solve people's RPG problems, and outside of "I'm curious about the reasons for this", there's no problem statement that requires designer reasons to answer. What the designers thought about a rule isn't really relevant to solving a problem, how to solve the problem is. Like it says in the Help Center, curiosity isn't a problem that we solve here. Also, as we established in the meta question about how to handle designer quotes, authoritative sources on their own are just sources, and not answers in themselves.
To illustrate this, I'm going to grab a couple well-rated [designer-reasons] questions at random from the tag.
Why is there a saving throw for each ability score?
This question is asking about how the Str, Cha, and Int ability score saving throws can be used in 5e. Quotes from the designers are relevant to the solution, since they can explain future plans for the edition, but the question isn't inherently about the designer reasoning. When asking the question, an equally-valid answer could have been "Oh, on such-and-such a page, this is all explained", without ever referring to the designer's reasoning. Since tags are not intended to restrict answers, applying the designer-reasons tag here didn't add anything to the question.
Have the developers of D&D 5e explained why they made Opportunity Attacks work the way they do?
This question explains a few issues that the querent sees with OAs in 5e, but really only asks about why the designers chose to implement them that way. This doesn't propose a problem to be solved, it just posits a complaint and asks people to explain why things are the way they are. As it says in the Help Center, “I’m curious if other people feel like I do.” is explicitly off-topic. If the querent flipped this around and proposed a 3.5-like homebrew instead, with a question on how that would affect 5e in general, that could be on-topic for the site.
I've looked through a number of other questions in this tag, and the pattern I've described here continues for all the ones that I've seen. There's the core of a solvable problem, but asking about the designer's reasoning doesn't actually help solve the problem.
It's not helpful to the site to have questions that ask us to solve the non-problem of curiosity, rather than asking us to help the actual problem of the game not working the way that people would like.