Wildgrowth Walker

• WWizards of the Coast2017-09-29
  Official Ruling
  If a creature leaves the battlefield before an effect instructs it to explore, it still explores. Effects that trigger when a creature you control explores, such as that of Wildgrowth Walker, trigger if appropriate.
• WWizards of the Coast2017-09-29
  Official Ruling
  If Wildgrowth Walker leaves the battlefield while its triggered ability is on the stack, you won’t put a +1/+1 counter on anything but you will gain 3 life.

Rulings via Scryfall