Photographic paper usually isn’t sold or labeled the way camera film is, so you typically won’t see ISO options in the same sense. If your goal is a much less light-sensitive material for longer pinhole exposures, one community suggestion is to use blueprint paper or diazo paper instead of standard darkroom paper.

These materials are comparatively insensitive, can be inexpensive, and are often available in larger sheet sizes. They’re commonly used for alternative-process photography and can give you the longer exposure times you want. Blueprint paper can also be made yourself if you want to experiment.

So while standard photographic paper generally isn’t marketed by ISO choices, switching to blueprint/diazo paper is a practical way to get a significantly slower recording material for pinhole work.