My previous research into retrocomputer OS updates has led me to the following list. For each retrocomputer OS, the date and version of the latest update released at time of posting is included. The link for farther information about the update is also included.
ProDOS 8 for Apple II, originally released Jan 1983
GS/OS for AppleIIGS, originally released Sep 1988
OS-9 for Tandy CoCo, originally released 1979
AmigaOS for Amiga, originally released Jul 1985
TOS for Atari ST, originally released Jun 1985
MS-DOS for IBM PC, originally released Aug 1981
Atari DOS for Atari 8-bit, originally released Sep 1979
RISC OS for RiscPC, originally released Jun 1987
MSX-DOS 2 for MSX, originally released 1988
OS/2 for IBM PC, originally released Apr 1987
MINIX for IBM PC, originally released 1987
- MINIX 3 is still currently maintained as of 2019. It requires an i586 CPU, which is closer to original than modern hardware is.
Linux for IBM PC, originally released Aug 1991
BeOS for IBM PC, originally released Oct 1995
Microsoft Windows 95 for IBM PC, originally released Aug 1995
Sinclair QL (and compatibles) SMSQ/E, originally released as Qdos 1984.
GEOS for C64/C128 and Apple II, originally released Jun 1986
PC/GEOS for PC clones, originally released in 1990
Digital Alpha machines
They don't have any paying customers with pre-Alpha machines.
Oric-1 / Oric Atmos
Sedoric has been maintained and improved :
Multics, first deployed in 1969
Multics 12.7 is now (2021-02) getting ready for release, a follow-on to Multics 12.6 which was released in 2017. It would surely run on the original hardware if you have a 6180 in your garage. Lacking that, it does run on an emulator ...