D&D is an RPG, a role playing games. Characters play a role in a story. The DM should administer the game so as to tell the best story with the players.
If a character would not miss out on using the ability, the player should assist the player in being aware it is an option. If the character might miss out on it, then there is no reason for the DM to intercede.
If it is something passive that the character does not need to decide to use, such as Danger Sense, then the reminder should be given. If ti is something that is partially active, such as Uncanny Dodge, then it is a judgment call. If it is something active that needs to be decided, such as when the DM notes the enemies are perfectly lined up for a Lightning Bolt, then no reminder is necessary - and perhaps is overstepping to offer.