Technically, it refers to obsolescent hardware -
Exactly. There is a lot of technology still in popular use that is no longer being built
into newer systems - those standards are considered "legacy" because support for
them is still found in in OS.
ISA is not legacy... It was replaced by local bus (VESA and microchannel) which was then
replaced by PCI, which was in turn partially replaced by AGP, and about to be widely replaced
by PCI-X and PCI-Express. That is 3 generations removed from the original standard.
There may be systems using ISA (I have a friend whose primary system still does), but the
technology itself has been long supplanted.
Serial and Parallel ports would be considered legacy, because USB and firewire have largely
replaced them, but you can find adapters to support USB to serial or parallel connections.
PS2 ports could still be considered current, because USB did not successfully replace them in
all cases, and you can still find new motherboards with PS2 built in IIRC.
USB (1.1 and 2.0) and IEEE1394 are current standards
Some technologies have both legacy and current components mixed.
IDE (PIO) is legacy, but still mostly supported by ATA-100 and ATA-133 controllers - which will
in turn be replaced by SATA (hopefully in the near future).
Newer SCSI standards are designed to support SCSI connections all the way back to the
original Asynchronous, narrow definition. (Not that you would want to connect a SCSI-1 device
on the same chain as a Ultra320 drive if you could help it).
"Legacy" in marketing terms means "stuff that we don't want to support anymore so we can
get you to buy a whole bunch of new stuff".