Ubuntu 16.04 is out of mainstream support https://wiki.ubuntu.com/Releases
This release was released in April 2016 and was maintained until April 2021.
However, Ubuntu 16.04 users can receive security patches for their system until April 2024.
To do this, use the connection to the Ubuntu Advantage program https://ubuntu.com/advantage and you can add 3 machines to one account (free for personal use) by receiving a key. All you need is an Ubuntu One account.
For commercial use, a paid subscription is offered, prices vary from $ 25 for a desktop, $ 75 for a virtual server and $ 225 for a physical server per year and above, more expensive prices mean not only updates, but also getting technical support through tickets and by phone.
In one of the latest updates to Ubuntu 16.04, the ua utility is installed on the system, with the help of which you connect to the service
To connect to the program for receiving updates, you need to log into your account, get a token and run the command
sudo ua attach _
You can check the connection status with the command
sudo ua status
Thus, you will receive updates for another three years until April 2024. It should also be noted that if you are an Ubuntu community member , you can connect up to 50 machines to the service.
It should be noted that for releases that are still relevant (ubuntu 18.04, 20.04) there is a livepatch service and with its help it is possible to obtain and install kernel security patches without rebooting the system. The terms of service are the same - 3 cars for free (and up to 50 cars for community members).