The Contract update for GTA Online revealed the real ending for Grand Theft Auto 5. In one of the missions Franklin is driving through a movie studio lot and says, "Man, sh**, I know one of the producers around here. I hope his ass ain't at work today." This is of course is not a direct reference to Michael De Santa, one of the three protagonists in Grand Theft Auto 5. But the character became a film producer in the end, and this line seems to confirm that Michael is still alive.
GTA 5 endings
There are three endings to single-player mode in GTA 5. We could:
Kill Michael;
Kill Trevor;
Save all protagonists.
Since Trevor was a part of GTA Online, we already knew that ending with his death is non canon. And now we know that Michael is still in Los Santos. So all heroes are alive and well.