Historical Rules · 1 April 2011
Rule 506.4d
Official source text from the 1 April 2011 Rules release
A permanent that's both a blocking creature and a planeswalker that's being attacked is removed from combat only if it stops being both a creature and a planeswalker. If it stops being one of those card types but continues to be the other, it continues to be either a blocking creature or a planeswalker that's being attacked, whichever is appropriate.