Historical Rules · 3 May 2019
Rule 506.4c
Official source text from the 3 May 2019 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.