Historical Rules · 1 May 2013
Rule 506.4c
Official source text from the 1 May 2013 Rules release
If a creature is attacking a planeswalker, removing that planeswalker from combat doesn't remove that creature from combat. It continues to be an attacking creature, although it is attacking neither a player nor a planeswalker. It may be blocked. If it is unblocked, it will deal no combat damage.