Ubuntu LTS has an additional 5 years of security support through Extended Security Maintenance thus giving LTS releases a full 10 year lifecycle. https://ubuntu.com/about/release-cycle
>ESM is available through an Ubuntu Advantage for Infrastructure subscription for physical servers, virtual machines, containers and desktops, and is free for personal use.
https://ubuntu.com/security/esm
Note that if you click through "personal use" means "up to 3 machines" and obviously doesn't apply to infrastructure. RHEL has free "personal use" subscriptions too, except they apply to up to 16 machines.
And also:
> Initially, free subscription is available for Ubuntu 14.04 LTS only.