Pure internet/spoken slang, popular since the 2010s. Used both as genuine praise ('legendary!') and ironically ('that's a new low'). Tone is everything - context decides which.
usage
Note the unusual reading 谁 = séi (a colloquial variant of shuí). Literally 'there's no one [like this] anymore' - meaning no one else can compare. Often paired with 真是 (zhēnshi) for emphasis.
