I would like to know if ST provides support for NuttX, the embedded OS that is POSIX compliant. I am wondering if there's an STMCube-like codebase of drivers or some sort of manual for getting started with NuttX.

[Tesla DeLorean]

I haven't seen them actively doing so. You'd perhaps have more luck at a NuttX forum

[Jennifer D]

Can you recommend one?

[Tesla DeLorean]

This is a bit of an uphill ski, I'm not the one with interest in the OS.

OS's tend to be driven by their adherents. ie I should port this to one of the most popular lines of micro-controllers in modern history..

Google suggests "NuttX Community Forum - NuttX Real-Time Operating System"

If you're not doing several $M business with ST they're probably not going to provide much direct engineering support to your ideas/projects