Question
Why is Roundcube 1.5.0/1.6.0 not available in Plesk for CentOS 7 and Ubuntu 18.04?
Answer
Roundcube version 1.5.0 breaks support of MySQL's old versions on CentOS 7 and Ubuntu 18.04.
Hence, Plesk does not provide Roundcube 1.5.0 and 1.6.0 versions for these systems.
As an alternative to use these Roundcube versions, consider migration to fresher OSes or dist-upgrade in case of Ubuntu-based system.