noun which may take the genitive case particle 'no'
not yet on sale; unreleased
Used for products, media, or tickets that have not yet been made available for purchase. Often appears in attributive form with の.
そのゲームはまだ未発売です。
That game is not yet on sale.
未発売の新商品の情報がリークされた。
Information about the unreleased new product was leaked.
発売予定 means 'scheduled to go on sale', implying a planned release date, while 未発売 simply means 'not yet on sale' without specifying a schedule.
Compound of 未 (mi, 'not yet') + 発売 (hatsubai, 'release for sale'). The reading みはつばい follows standard rendaku voicing of はつばい to はつばい in this combination.