In an update last year, the ability to specify any arbitrary callsign was removed. While I agree that random GA callsigns are messy and can become spammy (eg. UUUUUUUU), the update also removed any possibility of representing a flight number for an airline where the callsign is not in IF.
There are two possibilities for reintroducing the flight number: IATA and ICAO, but there are pros and cons to each approach:
ICAO flight number contains three letters for the airline, followed by numbers and maybe more letters. They are more realistic as these display on radar screens. On the other hand, they can also be used to spell out words (eg. BAL100N, not going to give inappropriate examples), which could cause the old issue of unprofessional callsigns.
IATA flight number contains two letters, or one letter and one number, for the airline, followed by numbers, and usually no more letters. This is less realistic, but would avoid the issue with spelling out words.
I believe the devs should consider these approaches and add one of them back into IF so we can fly routes without livery using the flight number instead of an aircraft registration. Please vote for this if you want them back.
(Note: this is not aiming to bring back VA callsigns)