A legacy font is an 8-bit font modified to include non-standard characters in the upper range. Often called "special fonts," these are not intercompatible - often within the same country or even for the same language(s).
Most often a legacy font was created for a Latin-based orthography that needed certain special characters (known in Unicode as extended characters) and/or diacritics used in some language's orthography, but not present in available fonts.
Unicode fonts, which are all intercompatible, are being used in place of legacy fonts.
At least one project, RIFAL, has been working on converting legacy fonts to Unicode.