Your Ubuntu-MATE 18.04 LTS release has plenty of life left.
Ubuntu-MATE 18.04 LTS is a long-term-support release, with 3 years of support starting with it's release in April 2018 (releases a yy.mm in format), so your support ends in April 2021.
The main Ubuntu 18.04 LTS has five years, but flavors come with 3 years of support. Only the first (April) release in a even year is LTS (ie. 14.04 LTS, 16.04 LTS, 18.04 LTS, 20.04 LTS), the normal releases have shorter 9 months of support life.
https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes
The 'main' archive of Ubuntu 18.04 LTS will be supported for 5 years
until April 2023. Ubuntu 18.04 LTS will be supported for 5 years for
Ubuntu Desktop, Ubuntu Server, and Ubuntu Core. Ubuntu Studio 18.04
will be supported for 9 months. All other flavors will be supported
for 3 years.