。
She kept up that habit until the day she died.
Literal
She [topic-は] that habit [object-を] death-until quit-not-did.
死ぬまで ('until she died,' literally 'until dying') uses まで attached to a verb to mean 'up to the point of [verb-ing].' Combined with the negated ~なかった ('didn't quit'), it expresses lifelong persistence: she continued the habit right up to the end. The phrasing leaves the habit's nature undetermined — it could be a beloved daily ritual, a stubborn vice, or a quiet practice carried for decades. The phrase 死ぬまで is also common in vows and declarations: 死ぬまで愛する ('I'll love you till the day I die').