NSF has guidance on their website for those who have, submitted, or writing NSF grants:
With regards to conference expenses incurred, refer to the following FAQ:
A conference has been canceled, but I have nonrefundable travel and/or hotel costs. Can these be charged to an NSF Conference or Travel grant?
NSF is currently working internally as well as with our federal partners on a number of proposal and award-related issues pertaining to COVID-19. NSF will communicate with the community about these issues and will provide guidance as further information becomes available. In the meantime, please continue to follow all relevant policies and procedures, including those of your organization, and apply those practices consistently.
We will be in touch with those who have outstanding expenses due to cancel travel in anticipation of CIG support as we learn more.