If a planeswalker is dealt lethal damage during combat does it die immediately or wait for state-based actions?

Planeswalkers don't die the instant they take damage. Instead, damage dealt to a planeswalker removes loyalty counters from it (one counter per point of damage), and then state-based actions (SBAs) are checked. A planeswalker is put into its owner's graveyard by an SBA if it has zero or fewer loyalty counters on it (CR 704.5i).

SBAs are checked at specific times: after each step and phase, and whenever a player would receive priority (CR 704.3). This means that during combat, after damage is dealt in the Combat Damage Step, SBAs are checked before any player gets priority — so the planeswalker is removed from the battlefield very quickly, but not instantaneously mid-step.

This timing distinction matters. For example, if a planeswalker would be dealt exactly lethal damage by two different sources simultaneously (like multiple blockers), both deal their damage at the same time, SBAs are then checked, and the planeswalker is removed once — it doesn't 'die' partway through so that the second source misses it.

Example: Your opponent attacks your Gideon Blackblade (3 loyalty counters) with a 5/5. During the Combat Damage Step, the 5/5 deals 5 damage, removing all 3 counters and leaving Gideon at -2 loyalty. SBAs are then checked, and Gideon is put into the graveyard at that point — not mid-combat, not before damage resolves.

Note: Prior to 2010, planeswalkers used a 'redirect damage' rule, but under current rules (since Commander 2019 rules updates), damage is dealt directly to the planeswalker, removing loyalty counters equal to damage dealt (CR 306.7).