The US Coast Guard has the following notice to mariners posted in mosts of its districts. No idea how many are affected, but its likely it would mostly affect those with older US-registered Inmarsat-C terminals whose owners haven't been billed in a while. Only know way to test it is to use it.
"On 22 April 2020 any shipboard Inmarsat-C mobile satellite terminal, including those used in the Global Maritime Distress and Safety System (GMDSS), and still commissioned using the Federal Communications Commission (FCC) as an Accounting Authority will be barred. Affected terminals will still be capable of transmitting distress alerts, receiving distress-priority messages from RCCs and receiving SafetyNET maritime safety information broadcasts. However, all other functions including Long Range Identification and Tracking (LRIT), piracy alerting and messaging will be disabled on that date. After 22 April, affected terminals can be identified by sending a standard C mail (e-mail) from the ship earth station and confirming its delivery.
"Further information is available by FCC Public Notice DA 19-315 (https://docs.fcc.gov/public/attachments/DA-19-315A1.pdf) and recommended action is provided in FCC Accounting Authority Transition Frequently Asked Questions (https://www.fcc.gov/accounting-autho...sked-questions). Ships carrying affected GMDSS satellite terminals will be non-complaint with US and SOLAS GMDSS regulations.