The Season 2 (aka Angel of Darkness) finale aired Aug. 9, 2020 on TNT.

This limited series was based on a series of crime novels by Caleb Carr, and there isn’t (yet) a third book to adapt at this time. Plus, with TNT out of the original scripted TV game… who’s to say where any third season would land?

https://tvline.com/lists/missing-tv-shows-renewed-cancelled/the-alienist-season-3-tnt/