The Japanese community's equivalent is 'old works' (旧作, kyuusaku), a term reflecting the technologically inferior FM Synthesis music & 16-bit colour graphics. 'PC-98' is named after the NEC PC-9801 computers on which they ran on.
If you installed the update on the PC in an earlier version and ran the update afterward, you will notice that the PC is no longer protected. Microsoft released updates only for Windows 10 version 1709 and earlier and has not incorporated the updates into Windows 10 version 1803.