https://en.wikipedia.org/wiki/Oracle_Solaris
Having postgres work on solaris is just a bit of a fuck you to oracle in my book!
AIX only runs on non-mainstream hardware (POWER), plus IBM isn’t as open as Oracle is with providing access to it. This makes it significantly harder to support than Solaris is
Added to that, Solaris and Linux are closer to begin with. Historically, Linux copied a number of its APIs from Solaris; AIX was far less of an influence. Years ago, I had to port some psuedoterminal management code from Linux to both Solaris and AIX. I only had to make some minor changes to the Linux code to make it work on Solaris; the changes I had to make to get it to work on AIX were much more extensive
I've been able to debug PostgreSQL issues reported by [closed] Solaris users by booting illumos inside a virtual machine on my laptop in minutes...
I want to keep AIX support, for, well mainly irrational nostalgic reasons -- PostgreSQL used to run on pretty much the entire Unix family tree, and I wrote lots of code on AIX for a decade. But they make it hard. I don't know why an OS vendor wouldn't make an image of an OS available to developers as conveniently as possible (it is even possible to boot recent AIX versions on QEMU if you have the patience... the hypothesis is that they might have done work to make that possible, 'cause it didn't work in earlier versions; but you can't get an image of OS, so shrug).
IBM Cloud does offer some free credits – at the moment US$1000 free VPC credits – but I'm not sure if you can use them on AIX (which isn't part of their main VPC offering, POWER is a separate offering), and even if you can, they aren't going to last forever.
If one is an open source developer doing this on one's own time, as opposed to a business – why spend US$50/month to get an AIX environment? Maybe, if a user of the project really wants AIX support, they could donate that to the project.
https://www.postgresql.org/message-id/flat/CY5PR11MB63921AB0...