It says in https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions that Ubuntu 18.04 can be reached through the ubuntu-18.04 or ubuntu-latest tags but shouldn’t ubuntu-latest be 19.10?
18.04 and 16.04 are Long Term Support (LTS) versions, which are more appropriate for this use case as they receive security and other updates for much longer than other releases. Using non-LTS releases would require users to switch much more frequently. The next one will be 20.04. See https://ubuntu.com/about/release-cycle
Still 18.04 should not be ubuntu-latest. ubuntu-lts probably - since the latest one is 19.10.
In this context, latest means “the most recent version provided by GitHub for use in workflows” and not “the most recent version released upstream”.