Slayed Necros
Void
What year did it start?
...and with which bands?
For modern retro thrash i would point to Municipal Waste in the early 2000's (2001-2003ish). For modern OSDM I would say around 2008 or 9 at the earliest with Entrails. Specifically modern OSDM not modern death metal of course
I'd argue that it always persisted in the underground with the likes of Pentacle, Sadistic Intent, Deteriorot and Mortem, but the revival definitely began before 2008 with bands like Repugnant, Slugathor, Deathevokation, Kaamos, etc.