Sforno on Leviticus 16:31/ספורנו על ויקרא ט״ז:ל״א

1In order to make this point even clearer the Torah wrote concerning this Day of Atonement, שבת שבתון היא לכם, this refers to the uniqueness of this day. A Sabbath day is usually honoured with food and drink, whereas the abstaining from physical indulgences i.e. a צום, fast day, is meant to put us in the good graces of G’d. (Isaiah 58,3) The message here is phrased differently but is the same. The self-denial of the Jew on that day results in the concept of the Sabbath being observed at its highest potential.

2חקת עולם, even at a time when there is no Temple, and hence no sacrificial service, the day of Atonement does not lose its function, and the Sabbath observance, i.e. abstention from work as well as the self-denial consisting of fasting for twenty four hours remains in effect.