Firstly I'd check your system has all security upgrades applied; as an upgraded bionic system will report itself as 18.04.6. The fridge notice of 18.04.6 release shows the date of the ISO release, but installed systems upgrade before that actual date.
Also please note flavors of Ubuntu come with 3 years of supported life, so Ubuntu-MATE 18.04 LTS is actually EOL; see the EOL annoucement on this discourse.
Yes packages that are common with Ubuntu Server/Desktop/Cloud/base come with 5 years of supported life; so a good deal of your system still gets security updates; but be aware it's not all of your system.
You can use ubuntu-support-status to confirm which packages get security-updates, and which do not, allowing you to assess the risk of your using a partially supported system if your system is on-line.