"If that feels like a long time between updates, remember that... "
...there also have been five TR (Technology Refresh) updates in said three years.
IBM has outlined a major update to the "I" operating system it offers for its Power servers. i 7.5, which will debut on May 10, supersedes the version 7.4 that appeared in April 2019. If that feels like a long time between updates, remember that servers packing IBM's POWER CPUs can also run IBM's own AIX or Linux – a variant …
You've been looking in the wrong place, then. eComStation and ArcaOS have been available since IBM stopped supporting OS/2, and ArcaOS is still under active support, according to Wikipedia.
Although OS/2 on Power was a thing internally (and I saw it running on an RS/6000 7020 40p back in the day), I don't believe that IBM actually offered it to customers.
I seriously doubt that even if you had a copy of the OS/2 for Power install media, that it would work on a Power 10 system, as there will not be the drivers for the PCIe adapters.
and ArcaOS is still under active support,
Latest news from the ArcaOS front was just over a week ago...
This venerable OS started life in 1978 and was christened "System/38" but by the time it got to high school age it was renamed OS/400 but it must have done something bad around 2004 as it souk anonymity as "i5/OS" or just plain "i" to its mates.
The whole AS/400 series were strange beasts featuring many unique architectural ideas some like addressing ram and all the disk storage as one single address space faded away others like concatenating file systems (libraries in OS/400 terms) are now mainstream and the basis of modern container systems like docker.