Any airlines receiving funds from CARES must maintain service to areas served prior to March 1, 2020.

https://www.transportation.gov/briefing-room/us-department-transportation-issues-show-cause-order

U.S. Department of Transportation Issues Show Cause Order

Tuesday, March 31, 2020

The U.S. Department of Transportation today issued a show cause order proposing parameters for initially implementing the authority granted to the Secretary of Transportation by Sections 4005 and 4114 of the Coronavirus Aid, Recovery, and Economic Security Act (the CARES Act). The order proposes that air carriers receiving financial assistance under the CARES Act maintain minimum air services on a nationwide basis, with some exceptions. Air carriers offering scheduled passenger service and all-cargo carriers would be covered by the proposed order; though, at this time, the Department proposes to not require service obligations of all-cargo carriers. The proposed order would not cover charter operations or air taxi operators. The proposal seeks to ensure that covered air carriers maintain a defined schedule of flights to communities they served prior to March 1. The Department proposes an initial term for the service obligations through September 30, 2020, which may be extended by the Department. The Department invites comments on this proposal by April 2.

The show cause order can be found at regulations.gov, docket DOT-OST-2020-0037. You can also view it here.