The Ubuntu LTS enablement (also called HWE or Hardware Enablement) stacks provide newer kernel and X support for existing Ubuntu LTS releases. These enablement stacks can be installed manually but are also available when installing with Ubuntu LTS point release media. These newer enablement stacks are meant for desktop and server and even recommended for cloud or virtual images. However, if one wants to remain on the original GA (General Availability) stacks, the options are:
Install from a previous 12.04.0/12.04.1/14.04.0/14.04.1/16.04.0/16.04.1/18.04.0/18.04.1 point release and update. Previous releases are archived at http://old-releases.ubuntu.com/
Perform an update or upgrade to an LTS release from a previous release.
Perform a network install using the netboot images rather than the new <release>-netboot images.