Which material "creeps" (plastic deformation) the least (or nothing at all) under pressure after being printed? I would like to build a standing shelf where the supports which hold each successive plank are 3D printed (to obtain special shapes).

However, I read that PLA flows under constant stress/pressure. Still, this doesn't stop the author of the article from using PLA for a hanging shelf, which obviously is subjected to constant negative pressure.

Which material suffers the least from plasticity/non elastic deformation under stress?

Answers with data for multiple materials are welcome.

I found that the phenomenon is called "creep" and is related to ISO 899, but I couldn't find any data for the common filament plastics and I don't know the theory behind it, so I'm not sure whether it's unavoidable or it appears after a threshold stress is reached.

Information: it's a living room shelf which will hold books and other stuff and is supposed to last a decade. I will of course add a safety factor and I could even fix the planks to the wall (in hidden places), but ideally the 3D printed material should have NO creep.
Your question can not be answered theoretically -- only empirically.

You need to print some trial brackets with materials of interest and measure them.  The question is: under what conditions should you test them?

The problem is that creep involves both compression and tension, and the behavior may be different.  It is also impossible to fully translate material specifications into component behavior without a really good model that includes the details of the infill, adhesion to peripheries, and all the microscopic detail of a real 3D-printed part.

The problem with typical PLA may be the low temperature.  Raising the temperature of a normal PLA printed object to 160°F (70°C) softens it to the point of nearly being limp.  I have used this for force-fitting PLA parts by warming a pot of water and placing parts in it for a few moments.  That temperature is hotter than your room, but a hot summer day in the sunshine could soften the part to the point of failure.

For anything load bearing, I would want a material with a higher plastic temperature.

There are PLA formulations which are annealed after printing.  This is claimed to allow the PLA to slowly recrystalize and become both stronger and usable at higher temperatures.  I don't have experience with this.

Depending on your printer, you can also consider using a higher temperature filament, such as ABS or PC (polycarbonate).  PET-G is a little better than PLA, but it softens at a lower temperature than ABS.

As important as the material itself is the anisotrophy of the printer parts.  Be sure to print the parts so that the major stresses are along the strongest axes, typically X and Y, and not along the weaker Z asis.  Choose your infill to contribute to the strength, and use plenty of it, or design the part so that the infill is not intended to contribute to the strength.