The Terraform provider supports setting up IP access lists for organization API keys with "mongodbatlas_access_list_api_key". For project API keys, however, there is no way of managing IP access lists with Terraform, and we have to do it manually using the MongoDB Atlas UI.
Terraforming this is valuable because it would allow us to document the why and the what for each IP address or CIDR block. Having it in code will also increase the visibility of these access lists to the rest of the team, as changes to project API key access lists will be code-reviewed like any other code. Ultimately, this should be good for MongoDB Atlas because it will increase the visibility and usage of this security feature.