7+ years to be exact. It's unfair to call Leaflet dead — it's still quite actively maintained, gets regular releases and has a huge community of both users and contributors, and Mapbox have always been supportive of me spending time on it during work hours.

What someone can see as less development activity is simply a sign of a mature product that doesn't need many new features and changes to remain useful — focusing on the core basic mapping needs has always been its goal, and it continues to adhere to it.