I tried to craft an honest, well thought out response that doesn't violate the "be nice" policy here. It was a Herculean effort, make no mistake.
The short answer is a resounding NO.
The longer answer is a bit more involved. While this question is put forth in a logical way, it is built several false premises, namely asserting opinions as fact, then building the argument on those opinions as if they were facts.
Being that we live in a post-binary-gender society
This is a sweeping generalization asserted as fact. There is no "we" and there is no society. SE is comprised of people from all over the world with differing customs, base languages, and societal norms. This false assertion alone makes the answer a NO. on the grounds that the assertion that there are as many as 63 genders is not accepted as even scientific fact, much less universally accepted in various cultures around the world.
the oxford dictionary states that it is grammatically correct to use non-gender specific pronouns.
This is an irrelevant appeal to authority and again, not the only dictionary of record. If we accept it as absolute, then all Americans, for example need to start spelling aluminum as "aluminium" defense as "defence", armor as "armour" et cetera ad nauesum. More to the point, the assertion that one thing may be correct does not negate the correctness of another thing. The fact that the dictionary asserts that the word hiccup is spelled "hiccup" does not negate the fact that hiccough is also a correct spelling for the word and even passes a spell-check.
However, some users get as far as upset when correcting their use of binary-gender pronouns
Again, this point is creating a false assertion as I outlined above. The assertion that one thing is correct does not mean that an alternative is incorrect. the so-called "corrections" being referenced are not actual corrections, but assertions of political opinions under the guise of correcting grammar which is not incorrect to begin with.
In Workplace posts, would it be considered offensive, and therefore flag-able, to use gender specific pronouns?
Again, a resounding NO! and for the following grounds.
- The assertion that gender specific pronouns are incorrect or offensive is entirely a subjective matter guided by personal politics.
- We have multiple cultures and multiple opinions on the matter, thus negating the entire assertion/
- It is an abuse of the flagging function
- It is a waste of moderators' time
- It has already been established that editing posts to change pronouns has, and will offend people, as acknowledged by this very question.
- Deliberately causing strife is a violation of the "be nice" policy.
- Cultural standards are not a cause to flag a post
- If we start making being offended, based not on the poster's intent, but solely reliant on the mindset of the one making a claim of being offended, literally nothing will ever be achieved in Workplace.
- It's an attempt to make an end run around the policy that the OP has final say in, and can reverse edits.