Will saplings drop if leaves are destroyed by piston?

Solution 1:

Leaves destroyed by a piston or pushed block can drop saplings and/or apples.

Keep in mind the drop rate of saplings is 5% (2.5% with jungle leaves), so a low yield should be expected.

It might be worth pointing out, that a TNT explosion has a 100% drop rate (no increased drop loss) since version 1.14 (snapshot 19w11a) as seen here, so destroying the leaves with carefully placed TNT could be more efficient than using multiple pistons.

The drop rates are taken from the Minecraft Wiki page for leaves.

Solution 2:

Yes they do.

See tree farm videos. Lot of them available on YouTube..