During last week's Smash Bros. Ultimate-centric Nintendo Direct, we learned Piranha Plant will be a playable fighter in the game, released post-launch and available for free to people who purchase the game in the first few months.
Nintendo has now confirmed on its official website that Piranha Plant is a 'free' download for players who purchase and register the game before January 31st 2019, and is expected to release around February 2019. February is also listed as the release month for the snappy one's amiibo.
Additionally the post confirms Piranha Plant is not one of the five fighters included in the 'fighters pass' paid DLC, though it will be available for purchase for players who did not register their copy of the game before the cut-off. Which seems a bit mean to be honest, but there you go.