You’re not allowed to go into a helicopter on NPS or Forest Service flights without leather boots and synthetics are not allowed, as far as I know. I’ve only taken the computer-based training for a job coming up in a week, though. Point being, leather still has a use and leather is still fine.
Source: IALSE23 (
https://www.doi.gov/sites/doi.gov/files/interagency-aviation-life-support-equiment-handbook-guide-v3.0.pdf#page10), Chapter 2.1, Section B, Item 4 (picture related)