No, it's not. It only seems common because the story chooses to focus on all the dragon slayers. There's literally millions of other mages that the story doesn't focus on. Also ancient spells just means magic from 400 years ago. It's not the same thing as lost magic, which is magic that has become rare. Now, dragon slaying magic is considered a lost magic too. But even if it wasn't, it would still be an ancient spell.Dragon slaying magic is like the most common magic in fairy tail Manga.
But after lullyby arc
We had gulana arc, phantom arc, heaven arc, fighting festival arc, os arc, edolas arc, tenrou arc, gmg arc, tartarus arc
And then finally in alverzec arc we hear flames of emotions again
That is all of the manga between these two mentions of flames of emotions.
And it's not like natsu didn't need powerup in those. He required them in every arc. But again no mention of flames of emotion. Even igneel didn't use or we didn't notice him using flames of emotion.
I think it wouldn't be far fetch to believe that mishma forgot about it.
Also, the number of arcs that has passed is completely irrelevant. Dragon slaying magic was explained all the way in chapter 1 as an enchantment that gives your body the properties of a dragon. And that's still true to this day. Natsu has been constantly using flames of emotion throughout the series. It's what his magic is.