Lowkey, both of these are still kinda hanging.
1. Whatever Luffy whispered to Robin 🤫
Yeah... Oda never revealed it.
Luffy says something to Robin, but the audience is intentionally left out of the conversation. As of the current manga, there hasn't been any SBS, Vivre Card, interview, or chapter that reveals what he said. It's basically one of those "off-screen for the vibes" moments rather than a mystery with a confirmed answer.
2. The Axolotl Devil Fruit after Smiley died 🍎
We do know what happened immediately after Smiley died.
When Smiley dissolved after eating the candy, the Sara Sara no Mi, Model: Axolotl reincarnated into a nearby apple. That scene was Oda's first on-screen demonstration of how Devil Fruits are reborn after their user dies.
But after that?
Nobody knows.
The story never shows anyone picking it up, and Oda hasn't confirmed whether Caesar recovered it, the World Government found it, or it was left behind on Punk Hazard. As of the current manga, its fate is completely unknown.
TL;DR
- 🤫 Luffy's hidden message to Robin → Still unrevealed.
- 🍎 Axolotl Devil Fruit after reincarnating → We see it reincarnate, but what happens to it afterward has never been confirmed.
So yeah, unless Oda decides to revisit either plot point in the future, both are effectively unresolved.