Yeah, Anything pre-WoD doesn’t have a mythic mode.
Any pre-WoD dungeon that does become a keystone only gets a mythic mode to support mythic +. That being said, with the changes they made this season, they will get a mythic mode but only for that season. For example, Grim Batol for TWW season 1. Once S2 begins, bye bye mythic Grim Batol. And heroic GB should scale back down to level 35. I say should because BFA world content (including normal dungeon and Islands) are still capped at level 60, even though they should be level 50. As BFA is no longer the default leveling expansion.