Q:

Which original retrocomputer OS's are still maintained and updated today, for original hardware?

I would like to know which original Operating Systems that run on retrocomputers are still being actively maintained and updated. This only applies to retrocomputer OS's that may still be used on original hardware.

Assuming the OS vendor has long-since dropped support, the updates would have to come from some community project which continues to develop the OS by adding modular enhancements, or by reverse-engineering the OS and releasing an enhanced version.
Modern replacements for the original OS that are in no way compatible
should not be included.
Updates should be able to run on the
original retrocomputer hardware without need for modern
hardware or emulators.
Updated ROMs are Ok, since this may be the pathway to update the OS.

A:

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

ProDOS v2.4.2 (Jan 2018)

GS/OS for AppleIIGS, originally released Sep 1988

GS/OS v6.0.4 (Jan 2017)

OS-9 for Tandy CoCo, originally released 1979

NitrOS-9 v3.3.0 (Apr 2014)

AmigaOS for Amiga, originally released Jul 1985

AmigaOS 3.1.4 (Oct 2018)
Classic WB v28 (Dec 2015)
AkReal v9 (Oct 2017)

TOS for Atari ST, originally released Jun 1985

FreeMiNT Open Source on Github (current)
EmuTOS Open Source on Github (current)

MS-DOS for IBM PC, originally released Aug 1981

FreeDOS v1.2 (Dec 2016)

Atari DOS for Atari 8-bit, originally released Sep 1979

SpartaDOS X v4.49 (Apr 2020)

RISC OS for RiscPC, originally released Jun 1987

RISC OS v5.24 (Dec 2018)

MSX-DOS 2 for MSX, originally released 1988

Nextor v2.1.0 (Aug 2020) - Source in GitHub

OS/2 for IBM PC, originally released Apr 1987

eComStation (eCS) v2.1 (May 2011)
Arca Noae ArcaOS 5.0.5 (June 2020)

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

Linux 5.2.9 released Aug 2019  It still supports (at least theoretically) an i486 which was already available at the time of original release.

BeOS for IBM PC, originally released Oct 1995

Haiku R1/beta1 (Sep 2018)

Microsoft Windows 95 for IBM PC, originally released Aug 1995

ReactOS v0.4.4 (Feb 2017)

Sinclair QL (and compatibles) SMSQ/E, originally released as Qdos 1984.

SMSQ/E 3.34

GEOS for C64/C128 and Apple II, originally released Jun 1986

GEOS Open Source on GitHub (current)

Digital Alpha machines

Digital/Compaq/HP/HPE/VSI OpenVMS on Alpha V8.4-2L2 

They don't have any paying customers with pre-Alpha machines.
Oric-1 / Oric Atmos
Sedoric has been maintained and improved :

Version 3 : https://www.oric.org/software/sedoric_v3_0-741.html
Version 4 : https://www.oric.org/software/sedoric_v4_0-2338.html

