Let’s clarify this. Traditionally Ubuntu proper has a 5yr LTS and will do again for 16.04. Most other flavours only apply for a 3yr LTS.
16.04 is our first properly official LTS and after some discussion with members of the Ubuntu Technical Board, we’ve decided to follow what most other flavours do which is a 3yr LTS.
What does this mean? We will support Ubuntu MATE 16.04 upto the 16.04.5 release. After that, the Ubuntu MATE bits won’t get updates and fixes but the underlying Ubuntu base will.