It was lost not destroyed.We saw what happens to Luke but he was a fairly big foreign object.However the fate of the light saber could be different

Legends says this

The lightsaber, still clutched by Luke Skywalker's hand, had become lodged in one of Cloud City's lower air shafts and eventually was recovered by a maintenance drone.

So it could end up anywhere.

It was lost not destroyed. We saw what happens to Luke but he was a fairly big foreign object picked up by the Cloud City filtering system. However the fate of the lightsaber might be different:

Legends says this

The lightsaber, still clutched by Luke Skywalker’s hand, had become lodged in one of Cloud City’s lower air shafts and eventually was recovered by a maintenance drone.

So it could have ended up anywhere.