Brad Figg from Canonical confirmed that Ubuntu 20.04 is being built on Linux 5.4 kernel, on the Ubuntu mailing list. Canonical’s decision was not a big surprise since Ubuntu 20.04 will be a long-term support release. Linux 5.5 kernel is the latest release but the latest LTS release is still Linux 5.4 kernel. Kernel developers will maintain Linux 5.4 until 2021. Obviously, using the latest LTS kernel was the best option for Canonical for their LTS distro to be able to support it with less effort for a long period of time.
Linux 5.4 LTS
Brad Figg from the Canonical team wrote,
“We are building the Focal / 20.04 LTS kernel based on the upstream 5.4 kernel. We have a version of this Ubuntu kernel in the -release pocket and continue to iterate and update it regularly while working towards the 20.04 release date.”