Kuro Ken! and Don't Go Overboard are doujinshi / 同人誌 (usually fanbooks) that feature the Haikyū!! pairings of Kenma & Kuro and Ukai & Takeda. They are hard yaoi (BL) scanlations and rated NC-17 or R-18. That is, not safe for work and illegal in some countries for those under eighteen.
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Canonically, Kuroo Tetsurō is seventeen and Kozume Kenma is sixteen. Both Japan and USA's legal restricted age of consent is eighteen. From what I understand, my state law age is sixteen where Japan's is thirteen--if both parties are under eighteen and consenting then it is legal. [versus a sixteen year old/eighteen year old, in which the legal adult would be prosecuted.]
However, this doesn't change the fact that Kuro Ken! features minors. From what I can tell the team is at the training camp so they should be around canonical age if this is true. Technically, this is considered shota as the characters are under-age.
