I think the "Legacy" stands for the encodings of underscore as left-arrow,
and circumflex (caret) as up-arrow.
There are other fonts that do this, as for example the PETSCII encoding
(on that same website) indicates the Commodore PET series of computers,
had the same encoding of left-arrow and up-arrow where modern ASCII has
its underscore and circumflex (caret).
Presumably a wide range of computers in the 1960's and 1970's had those
symbols on these positions.