Can you deal combat damage to a player and then redirect it to their planeswalker?

This used to be possible under older rules, but the rules changed in 2010. Under the current Comprehensive Rules, planeswalkers are a legal attack target just like players. When you declare attackers, you choose whether each creature is attacking a player, a planeswalker, or (in some formats) a battle — you cannot attack a player and then redirect damage to their planeswalker.

CR 508.1 governs the declaration of attackers, and CR 306.7 (planeswalkers) specifies that planeswalkers can be attacked directly. Once you declare your attacker and the target of that attack, damage goes to that declared target — there is no redirection step for combat damage.

The old 'planeswalker redirection rule' that let players redirect noncombat damage from a player to a planeswalker was also removed (as of Magic Origins/Magic 2015 era updates). Today, spells and abilities that deal damage to a planeswalker must target that planeswalker directly (CR 306.7).

Example: You attack with a 3/3 creature. During the declare attackers step you must choose: is this creature attacking your opponent directly, or is it attacking their Liliana of the Veil? If you choose the opponent, all 3 combat damage goes to the opponent — none can be moved to Liliana.

So to damage a planeswalker in combat, simply declare your creature as attacking that planeswalker during the declare attackers step (CR 508.1d).