"Vertical" as in a marketing channel for ads (disguised as a vaguely interesting website or publication).

In longer form: basically a codeword for "A once vaguely or perhaps genuinely interesting website / channel / magazine / whatever but which is now (through various nefarious means) in the possession of Megacorp X, which, in turn, couldn't begin to give a toss about the actual content (let alone the so-called "community" of once passionate viewers / readers), other than through the perception that they serve some marketing demographic Y, real or imagined, to whom, they would like to fancy, they can sell zillions and zillions of ads to."