What is the ruling on making up for the missed regular Sunnah prayers from past years, and are they waived during travel or illness?

A promise does not become a vow unless it is made with the intention [of being a vow]. If it is intended as a vow, then one is obligated to perform those Sunan (plural of Sunnah, referring to recommended acts), and they are not waived due to illness or travel. What was missed must be made up, whether with an excuse or not, and there is a difference of opinion regarding the necessity of expiation (kaffarah). However, if it was not made with the intention of being a vow, then making up [the missed acts] is not obligatory. Nevertheless, it is advised to maintain them because they are from the emphasized Sunan, and because fulfilling a promise is among the virtuous ethics.