Starboard tracks the bare minimum information necessary to keep the bot running. The only data that is really included are object IDs (message IDs, channel IDs, user IDs, etc.) as well as bot-specific information (how much XP a user has, how many stars a message has, etc.). Starboard does not store usernames, guild names, or message content.
Access to information such as joined guilds is restricted solely to developers of Starboard.