Back in April 2018, Ubuntu Studio 18.04 was released as a non-LTS (Long-Term Support) version, which limited its support cycle to end January 2019. This was due to a number of factors, from the involvement of the team members at the time to the number of team members.
In January 2019, the team came up with the idea for a Backports PPA of certain software to eliminate certain bugs and update the main packages (the ones that make Ubuntu Studio what it is). It was officially announced in February 2019.
As such, the Ubuntu Studio team no longer supports Ubuntu Studio 18.04 unless the Ubuntu Studio Backports PPA is added. Adding the Ubuntu Studio Backports PPA increases the support length of Ubuntu Studio 18.04 to 3 years total, with support ending in April 2021.
The Ubuntu Studio Backports PPA contains backported packages for all currently supported versions of Ubuntu Studio, with the exception of Ubuntu Studio 16.04 LTS which reaches end-of-life on April 25, 2019.