I've already raised flags on it - it has -4 votes, but isn't deleted.
The question is by a high reputation user here.
The question asks for a "Gender neutral alternative to Sir/Madam" and the answer suggests one. It's even backed up with a citation, an example of a use.
Thus it is an answer. If you don't think it's a useful answer, and the asker should really disregard it, then downvote it.
The "rude or abusive" flag is primarily intended for posts which are directly abusive of a particular person [an ad hominem attack*], or abusive of the system (thus cocking a snook at the entire community). The answer given does not fit either of those criteria. The fact that in certain circumstances using the answer might cause you to abuse someone by calling them "Thing" does not make the answer itself abusive. It's merely not useful.
A flag on the answer was declined in November 2019: "a moderator reviewed your flag, but found no evidence to support it." That's a canned response available to moderators with a radio button, as it's usually self-explanatory. However, perhaps this answer explains the lack of evidence found.
*For example, the various snide comments made against various American presidents, which I for one flag and remove on sight.
I've deleted this answer.
It's not about whether it's correct or incorrect, it's downright offensive. We don't allow offensive content on this site (with a very few specific exemptions for discussion of offensive things).
It seems that there's some disagreement of whether this is offensive or not - which... I don't agree with but I accept that it exists. To that end, I will offer that there is no value in retaining an answer that some consider offensive and that's downvoted so much and that lacks an actual example of usage - characters in a book named "Thing 1" and "Thing 2" is not the same as referring to an actual person on the phone or in person or in an email.
So, if you don't consider it offensive, it is, at minimum, not an answer.
Following that open interpretation, I've deleted it the normal way, not flag deleted it.
Looking at the comments, I assume that the answer was posted as a joke and not to make anyone feel bad.
However, the wording of the answer, when looked at in the context of the question, gives the impression that it’s making fun of (ie mocking) OP, people who want to be politically correct, or perhaps even people with gender-neutral voices. And the core of the answer (being a joke) isn’t a good answer to the question, so it’s not really worth trying to reword it.
That’s why I think it should be deleted (as has been done).
If the question was looking for an insult, the answer may have been ok—with additional references. Such a question should be worded as to avoid bigotry though.