The United Nations has a number of different functions as a collection of countries across the world working under one banner, with the creation of "The Office of the United Nations High Commissioner for Human Rights' Committee for the Rights of the Child" being one of them. Creating a number of guidelines to help put a stop to the "sale of children, child prostitution and child pornography", the U.N. has focused recent efforts on the sexualization of minors in manga and whether or not these depictions could potentially be causing harm to society at large, potentially harming minors themselves.
The United Nations released a statement with regards to their concerns regarding the portrayal of minors in sexual situations when it comes to manga being drawn across the world, which you can read below if you want a better look into the thought process of the nations on the committee:
"The Committee is deeply concerned about the large amount of online and offline material, including drawings and virtual representations, depicting non-existing children or persons appearing to be children involved in sexually explicit conduct, and about the serious effect that such material can have on children's right to dignity and protection. The Committee encourages States parties to include in their legal provisions regarding child sexual abuse material (child pornography) representations of non-existing children or of persons appearing to be children, in particular when such representations are used as part of a process to sexually exploit children."
The Ministry of Foreign Affairs in Japan had this to say in response, believing that the committee should have discussed the matter with Japan and other countries that may be affected:
"It is disappointing that these guidelines were released without the Committee having carried out sufficient discussion among the countries, including our country, that are affected by the Optional Protocol."
The Committee for the Rights of the Child was originally created in 1989 and has been creating a number of guidelines with regards to protecting children the world over. The U.S. had originally offered resistance to the guidelines for similar reasons as Japan.
The UN's recommendation is just that without any legal recourse, and no word has been given on whether the guidelines will be used to push new laws down the line. For now, the fandom will have to wait and see how this suggested ban is treated in the future and whether it alters upcoming anime projects.