From Barry's description, the Injustice protocols are similar to the contingency plans that Batman devised in Justice League: Doom (2012) to neutralize each member of the Justice League should the need ever arise; he considered their powers, both individually and collectively, too dangerous to lack a failsafe against any potential misuse, whether intentional or not.
The place that Eobard and Iris are celebrating their partnership at is a nod to Eobard's younger brother in the comics, Robern Thawne. Eobard also erases Robern from existence, like he's doing with Barry's legacy here.
As Ryan Wilder from Batwoman (2019), this is Javicia Leslie's first Arrowverse crossover appearance. She is also the only guest star from a show still on the air; though DC's Legends of Tomorrow (2016) is also still airing, Brandon Routh, who played Ray Palmer in Part 1, left the show in season 5.
Jefferson mentions that the Injustice protocols were put in place "after Faust," implying an off-screen encounter with the sorcerer Felix Faust. In the comics, Faust was a member of the first incarnation of the Injustice League.
Despite this episode being the third episode of Season 8, it was the first to be filmed during production.