s there a technical name for a character that is mentioned by name but is never present in a story?
A character that is discussed, but does not appear in the work is called an "unseen character".
From wikipedia: "An unseen character in theatre, comics, film, or television, or silent character in radio or literature, is a character that is mentioned but not directly known to the audience, but who advances the action of the plot in a significant way, and whose absence enhances their effect on the plot."
The article also uses the terms: offstage character and invisible character which may have slight nuances in how they have been dramatized, but are generally interchangeable (an invisible character may be implied to be present, voiced from offstage or reacted to by other characters in pantomime).
Is "estranged" or "offscreen" character good enough?
I would not use these terms.
Estranged is a relationship status to other characters. It does not imply the audience's experience.
Offscreen (originally offstage) is a production (stage) direction, and implies a temporary condition. Characters and objects that have previously been offstage can be moved onstage, and back again.
In the case of film, an offscreen direction can literally last a few frames within a scene where the subject is present but the camera is pointed at something else, as in 2-person dialog where one character's response is heard "offscreen".