Ubuntu 16.04 LTS was released in 2016-April with 5 years of standard supported life. The announcement/release notes said
Maintenance updates will be provided for 5 years for Ubuntu Desktop, Ubuntu Server, Ubuntu Cloud, Ubuntu Core, and Ubuntu Kylin. All the remaining flavours will be supported for 3 years.
That meant Ubuntu 16.04 LTS reached EOSS in April 2021, with Ubuntu-MATE (being a flavour) reaching EOL in April 2019.
If you want an easier ride of upgrades, I'd suggest using one of the supported paths & also upgrading within the timelines the release says is supported (thus QA-tested for!).
Warnings of EOL (inc. EOSS) go out six weeks before EOL (eg. here) and are actually posted in many ways and using different methods. You could always have run ubuntu-support-status on your system anytime for details anyway.
I'd have just opted for a upgrade via re-install (ie. repair type of installation where you install the newer version instead of the same version as the install type was intended for), alas with a six year jump, you'd have to check for each program where data matters to you, to ensure no data-adjustment steps were missed in those six years. The checks done by packagers/developers assume you use supported upgrade moves & not the jump you made.