Old SoundFonts continue to function because the .sf2 file format is a standardized container for MIDI-mapped samples. As long as a software "player" can read the instrument definitions and sample data, the age of the file is irrelevant.
: High-end samplers like Native Instruments Kontakt can often import SF2 files directly, though results may vary with complex modulation. Where to Find Classic SoundFonts old+soundfonts+work
by Plogue: A highly stable, free player that converts SF2 files into the modern SFZ format on the fly. Phenome : A straightforward, multi-timbral VST player. Old SoundFonts continue to function because the
"On the Perceptual Artifacts of Sample-rate Conversion in Digital Audio" old+soundfonts+work