The official update v3.0.3 for Super Mario Maker 2 is a minor maintenance release primarily focused on system stability and improving the overall user experience. It does not introduce new gameplay elements like the major 3.0.0 "World Maker" update, which added features like the Super Mario Bros. 2 Mushroom and the ability to create entire worlds. Update Highlights
Because the official notes are vague, dataminers and community members have investigated the code changes: Security & Stability: Dataminers like OatmealDome super mario maker 2 nsp actualizacion 303
Check out the breakdown of the major content that v3.0.3 maintains from the final content update: The official update v3