It is lowercase only. But lowercase in number case. The upper and lower case is distinguished based on alignment in this, where in text case it is based on shape and/or size.
Edit: my use of the words number case and letter case does not look like the standard words. But the concept still exist.
Check this: https://totallytype.com/figures.php
Wikipedia says that they are sometimes referred to as lowercase.
Most font packages call it old style figures.
In typography class we were taught to call the lining and non-lining figures.
It is lowercase only. But lowercase in number case. The upper and lower case is distinguished based on alignment in this, where in text case it is based on shape and/or size.
Edit: my use of the words number case and letter case does not look like the standard words. But the concept still exist. Check this: https://totallytype.com/figures.php