The term Legacy refers to all content, especially Units released before Genesis and the Special Forces, namely those released before March 19th, 2015. They can be distinguished from current units by having no unit type and stats much lower than Special Forces and Faction Units. Furthermore, in the Units UI, the Legacy units can be distinguished by being put in a gray background which does not match the theme of the UI.
They are called Legacy because KIXEYE decided that these units will no longer be in active development, and has been . Their stats will never be changed and no new tech will be provided to put them back into relevance.
They still show up occasionally in War Commander.