In the comments to the above linked question, depperm correctly points out that, as written, the help center makes all "Is X a sin?" questions off-topic.
However, there are questions that are not constructive for the format of this site. These include questions asking for:
is "X" a sin questions that ask whether a certain action or belief is sinful
"Truth" questions that do not focus on what a specific group of people teaches
The separation of those two bullets is, I believe, the issue. In that same post CuriousDanni suggests the question is on-topic due to the focus on one denomination, which is not technically specified in the help center.
I believe it is legitimately possible to answer "Is X a sin?" questions when properly formatted and directed toward one tradition or denomination. However, I also recognize that human habit will be to not give a documented answer, but advice.
Below I've posted two answers seeking community consensus.
One in favor of adding "...that do not focus on what a specific denomination or tradition teaches" and making "Is X a sin?" questions on-topic in that context.
One against the change and leaving "Is X a sin?" questions permanently off-topic.