Draft -- pending real legal review before any public launch. Provided as-is for early testers.
Your email address, a hashed (never plain-text) password, an optional display name and birthdate (used only to confirm you meet the minimum age requirement), and whatever decks/collection entries you create.
Solely to run the app for you: keeping you signed in, saving your decks/collection, and sending account-related email (verification, and if you ask for it, a password reset). Nothing is sold, and nothing is shared with advertisers.
Card lookups are served from a local cache built from Scryfall's public data; combo lookups similarly from Commander Spellbook's public data. Outbound email currently goes through a sandboxed test provider during early testing, not a live mail service.
One session cookie keeps you logged in. It's not used for tracking or advertising.
Since this is an early testing build, email the person who invited you to have your account and data removed.