Security support for Ubuntu GNOME 16.04 LTS is supposed to end in April 2019, that is, this month. However, I couldn’t find any information regarding when exactly in April this will happen. On what day will the security support end?
30-April-2019 for all GNOME related GUI parts, plus anything else from the 'universe' (or community-supported) repository.
End of April 2021 for all 'main' repository software, which includes kernel & some programs provided in Ubuntu-GNOME 16.04.
For more information on your actual install, please use