Where? In what contexts? It's not required for houses or town houses where I'm from at least. From the pictures I guess it's mostly relevant to larger buildings?

"Over 90% of the homes that are built in the United States each year are constructed using wood-frame construction"

https://awc.org/codes-standards/firesafety

And close to 100% of those are built with garden variety untreated fir lumber.

This is too general a blanket statement to be true. Plenty of wood in construction is not treated for fire. Fire ratings are generally done for assemblies, not components of assemblies, and there are ways to build fire rated assemblies without treated wood. Also, not all assemblies need fire ratings. Sometimes its just the exterior walls adjacent to neighbouring structures, etc.