According to here
16.04 is supported to April 2019 which differs from
ubuntu-support-status
Support status summary of ‘k’:
You have 62 packages (2.2%) supported until April 2021 (Community - 5y)
You have 1844 packages (66.6%) supported until April 2021 (Canonical - 5y)
You have 658 packages (23.8%) supported until April 2019 (Community - 3y)
I think the date mentioned at the download page is the ubuntu mate specific guaranteed support date. That does not mean that some cannot be supported for longer.
From XenialXerus/ReleaseNotes - Ubuntu Wiki
Support lifespan
Ubuntu 16.04 LTS will be supported for 5 years for Ubuntu Desktop, Ubuntu Server, Ubuntu Core, and Ubuntu Kylin. All other flavours will be supported for 3 years.
So it seems the Ubuntu MATE-specific packages are the 23.9% that are supported for three years, while the others are packages supported by Canonical as part of Ubuntu Desktop. (The 2.2% that are supported by the community for five years I'm less sure about.)
thanks.
Maybe the 2.2% are some outside packages I installed.
I remember manually adding some repositories.
I think Seamonkey was not there for a while.
I see no reason to go to 18.04 if everything is working fine on 16.04.