Questions pertaining to systematic theology are off-topic for this site. The reason the question about Purgatory was put on hold has nothing to do with differing ideas of canon. The issue is that the question is about Purgatory, not about the Bible (however that might be defined).*
Notice the tagging on the question - 1-corinthians, revelation, 2-samuel - and yet the question doesn't ask anything about any verse in any of those books. If the question instead asked about what 1 Corinthians 3 means when it talks about "escaping as one through the fire", that would be on-topic. Likewise, if the question was about 2-maccabees itself and the meaning of a verse, I think it would be likely to stick even though it isn't in the 66 books of the protestant canon.
I hope that clarifies things. Our goal is not to restrict this site to a particular group's canon per se. But we do draw lines around how far from the examination of the Bible one is allowed to get, and questions about theology proper are outside those lines.
I will note that a number of the more active people here are protestant and might not have as much expertise or familiarity with the books accepted in the Catholic and Orthodox traditions. It might take longer to get an answer on questions about some of those books. However, asking those questions might also be a good way to attract wider attention so that this site does become a place where people get good answers on those questions in a timely manner as well.
* (Also, we've decided that the site is not a crowd-sourced Bible search engine, and so we have said that questions asking for "where the Bible talks about x" are off-topic as well.)