What is mandatory upon one who has missed days of Ramadan due to menstruation several years ago and has not made them up until now?

The missed days of Ramadan must be made up before the advent of the next Ramadan, based on the hadith of Aisha, may Allah be pleased with her.

The ruling on delaying the make-up until the next Ramadan varies depending on the situation: 1. Delay with a valid excuse: The one who delays is not sinful, and only owes making up the days. 2. Delay without a valid excuse: The one who delays is sinful. And he must make up the days. The imams differed on the obligation of feeding the poor (Kaffara) along with making up the days: Malik, Shafi'i, and Ahmad: Feeding the poor (Kaffara) is obligatory along with making up the days. Abu Hanifa: Feeding the poor (Kaffara) is not obligatory. This is also the choice of Al-Bukhari and Ibn Uthaymin, based on the verse: ﴿a prescribed number of other days﴾, and the absence of any mention of feeding the poor.

The most preponderant opinion is that only making up the days is obligatory, with the feeding of the poor being recommended as a precaution. One who delays making up the days without a valid excuse must repent to Allah.