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.
Yes, we are aware of what you summed up re. Debian support, Buster does have our attention. We are working on the x86 version to start, but have struck a number of issues during the upgrade process and unfortunately, due to our limited resources, have not been able to solve all of them yet. We did made progress the last couple of weeks, but it is too early to commit to a ETA yet, so no need to ask.