Frame challenges have long been considered acceptable here at worldbuilding.SE However, I believe many users don't understand what a frame challenge is or how to appropriate present a challenge as a legitimate answer to a question.
Though I've seen this come up a number of times in the past, the most recent example was this answer to A Decoy Planet made of expanded polystyrene. How big can I make it? The example is especially relevant because the respondent provided a wonderful alternative idea, one that was appreciated by the OP. However, and most especially thanks to the OP's use of the hard-science tag, the proposed answer did not answer the question. But, did it represent an appropriate frame challenge?
My proposal, for your consideration, is no. It did not. No matter how useful, I believe the answer in its present form is not an answer and would have been better submitted as a comment to the question.
Formally: I propose that frame challenges are an acceptable way to answer any question posted to this site. As a definition: a frame challenge is any assertion that challenges the premise or underpinnings of a question.
I propose that an acceptable frame challenge must fundamentally express the idea:
What you're asking for won't work because of X, but you could alternatively consider Y
I'm not looking to impose a specific form (syntax, format, etc.) as much as I'm proposing a philosophy. (A) A frame challenge must of necessity contain an explanation of why the respondent believes answering the question as proposed by the OP isn't useful, relevant, or meaningful. However, (B) A frame challenge need not offer an alternative solution.
I propose that a frame challenge must explain the challenge and not only offer an alternative.
I further propose that a frame challenge must meet the obligations of the tags associated with the question. In other words, if the question is tagged hard-science, the frame challenge must meet that tag's mandate or fail as a frame challenge.
I propose that when reviewing answers, Worldbuilding.SE users should consider a missing frame challenge as justification to flag the answer as "not an answer." They should consider a poor frame challenge (e.g., one that doesn't meet the expectations of the question's tags or is simply a lazy effort to rationalize commenting rather than answering) as a downvotable offense.
Finally, I propose this warning: frame challenges are not an excuse to abuse Stack Exchange's answering system. A frame challenge must be more useful than a comment or it must be posted as a comment (see previous paragraph about reviewing the answer). This warning is not meant to be anything more official than the community-approved existence of this paragraph.