From what I know there are a number of legal complications/liabilities to hosting NSFW content. For most people hosting instances it’s just not worth the hassle given that users can still interact with the NSFW communities on the dedicated instance.
That kind of discrimination is generally illegal, even after the recent supreme court case.
What the ruling says is that some kinds of business, such as designing a website, decorating a cake, or writing a song, for example, are considered speech. In those cases the right of the designer/decorator/songwriter to control their speech takes precedence.
However, this doesn’t mean you can kick someone out of your restaurant for being Jewish or refuse to make a non-marriage related website because a client is gay. It’s only cases where speech is involved.
15 years?! That happened in 2018 lmao