Can you assign combat damage to a planeswalker instead of a player if it's not being attacked?

Combat damage assignment follows the target declared during the declare attackers step. An attacker's combat damage must be assigned to the player, planeswalker, or battle it was declared as attacking — not freely redirected to any other permanent or player (CR 510.1).

Previously (before the 2010 rules update), there was a redirection mechanic allowing players to redirect damage from a player to a planeswalker that player controlled. That rule no longer exists. Under current rules, attackers are declared as attacking a specific planeswalker directly (CR 508.1), and only that planeswalker receives combat damage from that attacker.

So if a creature was declared attacking Player A, its combat damage goes to Player A. You cannot choose to instead assign that damage to a planeswalker Player A controls, even if you wanted to.

Example: Your opponent controls Jace, the Mind Sculptor. You attack your opponent with a 3/3 creature rather than declaring it as attacking Jace. When combat damage is dealt, you cannot assign any of the 3 damage to Jace — it all goes to your opponent, because that is who the creature is attacking.

If you want a creature's combat damage to hit a planeswalker, you must declare that creature as attacking the planeswalker specifically during the declare attackers step (CR 508.1, 510.1).