Whether it belongs on Stack Exchange is a normative question rather than a legal one, but my personal opinion is that the linked question is acceptable.
The Question includes some discussion of the law, at least by analogy. But Stack Exchange is completely allowed to take a narrower view of what is appropriate or even what is permitted than the law necessarily does. At its heart, this is a normative question for the community. Also, this is the type of normative question where reasonable minds can disagree.
With that said, my opinion is that this question is perfectly acceptable. We are talking about mooning someone in a way that is childish, but kind of funny in a childish way. My discussion of the law below gets far more graphic (by necessity) than the original post that spawned this question ever did.
Also, this site talks about in-game sexual assault all the time. Now of course, those discussions tend to center either on the best ways to avoid it, the best ways to handle it maturely when it fits in a mature story, or how to recover when something that should never have happened did happen. I hope to never see a question about how to actually optimize in-game sexual assault on this site. But, this is optimizing an in-game childish prank, not in-game sexual assault as that term is usually defined.
I think the linked question is appropriate for this site, but I acknowledge that reasonable minds could differ. Also, to the extent we do want to use the law as an analogy, this would not constitute sexual assault in any jurisdiction I am familiar with. It might constitute indecent exposure, but it would not rise even to that level in the jurisdictions I am familiar with.
This is not sexual assault as it is defined in most USA jurisdictions.
As a prelude to the legal discussion, I am a lawyer. But I am only licensed in Nevada, and I primarily practice civil appeals and real estate law. Nothing in this post, or anything I post on Stack Exchange should be considered legal advice.
I am not certain this should really be addressed as a legal matter, but this question directly invokes the law at least as an analogy. While I have not done a complete survey of the entire United States much less the entire world, in every jurisdiction I am familiar with, mooning someone could not be considered sexual assault. In many jurisdictions, including Nevada the legal phrase "sexual assault" is synonymous or nearly synonymous with rape.
In Nevada for instance, the definition is located in NRS 200.366. That is a rather long statute, but the core of it is:
1. A person is guilty of sexual assault if the person:
(a) Subjects another person to sexual penetration, or forces another person > to make a sexual penetration on themselves or
another, or on a beast, against the will of the victim or under
conditions in which the perpetrator knows or should know that the
victim is mentally or physically incapable of resisting or
understanding the nature of the perpetrator’s conduct; or
(b) Commits a sexual penetration upon a child under the age of 14 years or causes a child under the age of 14 years to make a sexual
penetration on themselves or another, or on a beast.
Notably, Nevada does not have a separate law by the title of rape and essentially uses sexual assault to address what many other states would call rape. California does have a separate offense of rape and instead has sexual battery as a lesser crime that involves either touching someone's "intimate parts" or forcing the other person to touch your intimate parts against their will. California's statute is even longer and hard to easily take a core quote from, but it is located at CPC 243.4.
This probably isn't indecent exposure in most parts of the United States
The definition of indecent exposure is highly variable throughout the United States and even more variable when you consider the world. In Nevada, at least as the statute is interpreted by the Nevada Supreme Court, indecent exposure requires exposure of the genitals or actual anus. State v. Castaneda, 126 Nev. 478 (2010). Exposure of the buttocks alone is not enough for indecent exposure. Id. at 489. California's interpretation of its indecent exposure laws similarly requires exposure of the genitals. People v. Massicot, 97 Cal. App. 4th 920, 924, 118 Cal. Rptr. 2d 705, 707-08 (2002).
Notably, exposure which does not by itself constitute "indecent exposure" can still be breach of peace or disturbing the peace, but in most places that requires malice. Also, I think this is a good time to emphasize that none of this is legal advice. And as a real side note, while I have not done a complete analysis, I think the First Amendment would likely prevent application of an indecent exposure law as long as the exposure had a genuine expressive component and did not rise to the level of also breaching the peace. In other words, mooning someone to show contempt would probably have First Amendment protections in the U.S. under most circumstances even if some indecent exposure law would theoretically forbid it.