In Avengers: Endgame, the titular heroes inadvertently take a 2014 version of Thanos with them to 2023, where they face off against the Mad Titan and his vast intergalactic army, but there are a couple of notable absentees from his evil horde. Since this Thanos is from 2014, he's the Thanos from when Guardians of the Galaxy takes place; therefore, Lee Pace's Ronan and Djimon Hounsou's Korath are still working for him.
Given that Thanos brings the Sanctuary II, the Black Order, the Chitauri, Chitauri Gorillas, Outriders, Sakaarans and Leviathans along for the final confrontation, it doesn't make sense that Ronan and Korath aren't there. At that point in 2014, they are two of his most prominent henchman. Furthermore, the aforementioned Sakaarans who do join in the battle are actually commanded by Ronan in Guardians of the Galaxy.
Moreover, their specific task while working for Thanos is to bring him the orb which contains the Power Stone. Given that Thanos arrives in 2023 knowing the Avengers took the Power Stone, it's even more confusing that he doesn't bring those along who are prepared to steal back the orb in the first place.
While this is confusing, there are a few potential explanations. For starters, it's established in Guardians of the Galaxy that Ronan struck a deal with Thanos in exchange for the orb. If he gives Thanos the orb, the Mad Titan will destroy Xandar. However, with Thanos going to the future, Ronan likely believes their deal has fallen through and leaves with Korath, his subordinate, since he no longer has a stake in the fight.
Another explanation is that this version of Ronan knows exactly what is inside the orb, unlike in the original timeline. It isn't until halfway through Guardians of the Galaxy that Ronan realizes the true potential of the Power Stone, at which point his agenda changes, using it himself as opposed to handing it over to Thanos. However, if the events that unfold in Avengers: Endgame give him this information earlier, he may have realized that Thanos would've gotten the Power Stone before him by traveling to 2023, so he chooses to distance himself from the Mad Titan for his well being.
While they are not present, one can imagine how their inclusion could've changed things for Endgame's final battle. While Korath would most likely have done as he was instructed, Ronan is a different story. He is already power hungry in Guardians of the Galaxy, so giving him the opportunity to potentially wield all six Infinity Stones would've been dangerous. Given his animosity over serving Thanos and his desire for power, he would've likely gone after the gauntlet himself if the opportunity presented itself.
With that in mind, his presence may also have resulted in some in-fighting amongst Thanos' ranks. If, for example, Ebony Maw or Cull Obsidian saw Ronan trying to defy their father's orders, they may have had to attack him instead of the heroes, which could have resulted in a quicker defeat for the Mad Titan's forces.
Essentially, while both Ronan and Korath are undoubtedly conspicuous in their absence, their presence in the Battle of Earth would probably have had even more of an impact than some would think. Either way, they would certainly have been interesting additions to a battle that was already a who's who of the Marvel Cinematic Universe.