Next years Eternals apparently went through many different changes before, and during, filming. Some of those changes most likely included character changes as casting information prior to the filming was very different than what we know so far about the movie. For example, the role that became Dane Whitman/Black Knight was originally being cast as a female archaeologist with the codename of “Karen“, and Starfox appeared in some of the early casting breakdowns.
But Starfox is too big of a character to just leave discarded. Marvel Studios still has plans to use the character as some point in a future MCU project. There are a lot of cosmic options on the way where one-time Avenger could show up, such as the Secret Invasion series that’s reportedly in the works or even something like Captain Marvel 2.
Starfox, aka Eros of Titan, is the brother of Thanos, and with future MCU movies dealing with the aftermath of Avengers: Endgame; it makes a lot of sense to bring in Thanos’ family (his father was previously named in Infinity War) to remind people of the Infinity Saga.
If he was indeed cut from Eternals, that also makes sense as could be given a bigger spotlight in a future project; as opposed to being overshadowed by that huge cast of characters we’ll see on the screen in February.