Here's how this breaks down:
- We do not, in point of fact, have a problem with answers talking about designer reasons.
- We do have a problem with answers making big claims without proportionate citation, and with misleading answers. This usually gets corrected by downvoting.
Questions asking about designer reasons were banned because, simply, we saw a ton of questions asking for designer reasons in D&D, and every single time we saw loads of answers that fit the second bullet point: people looked at D&D's rules, reverse-engineered some speculation about designer intent, and then presented their speculation as concrete fact. Thus we'd regularly wind up with multiple different answers, all doing this, all contradicting each other, not a single designer citation to be seen among them. Voters were not responding with downvotes, and moderators deleting things only made people angry. There was almost never an actual citation to be found (because D&D's designers rarely share the thought process behind a mechanic or piece of content) but folks could not abide the void; they wanted to answer.
It was a farcical breakdown of our system every time. We simply concluded that kind of question was not workable in our system and deemed them off topic.
Notably, however, not off topic because of the design reasons themselves.
To some extent, problems with answers can help identify problems with the question. Is a question drawing tons of purely opinion-based answers where good subjective is breaking down? That might help us notice it's an opinion-based question that needs to be closed and revised. (But maybe there's just a few low-quality answers we need to downvote and it's got nothing to do with the question itself.)
However in the case someone asked a good question and got a good answer that also cites designer reasoning... that's just a success of the system. Mission accomplished. No problems here. Designer reasoning was not itself the problem before and it isn't a problem here. Although, do keep an eye out for whether they're doing the reverse-engineered speculation thing or actually citing the designers.
It so happens that for some of our games that's relatively normal: design questions about adjusting Fate Core can often poke into the “why” of the current state of the game, for example, because the authors have just written so much about their design processes and reasoning.
If the question gets multiple answers all seeking to speculate about designer reasoning with no citation, then we may look to see if we have a bit of a problem in the question itself and if it needs revision. (Or maybe there's just a few low-quality answers that need downvotes.)