1. Ubuntu decided not to fix CVE-2018-15919.
Reference
https://ubuntu.com/security/CVE-2018-15919
2. Focal version is not vulnerable, therefore SSB 7 LTS is not vulnerable.
Bionic is fixed in SSH version 7.6p1 and SSB 6.0.0 LTS is shipped with it therefore every SSB 6 LTS has this security update.
Reference
https://ubuntu.com/security/CVE-2018-15473