Volumio is based on Debian Jessie, released in 2015-04-25, and end-of-lifed over a year ago (2018-06-17). Jessie does have another 9 months on LTS (EOL ~2020-06), but that’s supposed to give users time to migrate, not give developers an excuse to procrastinate further.
When will Volumio move to a more recent Debian release?
Stretch has been out for over 2 years; Buster was released in July. Raspbian Buster was released 2019-07-10.