IEEE MMSP 2021 timeline has been confirmed:
Special Session / Grand Challenge proposal submission: 30th April 2021
Regular and Special Session paper submission: 28th May 2021 extended 11th June 2021
Notification of paper acceptance: 16th July 2021 updated 23rd July 2021
Demo paper submission: 13th August 2021
You can find the full list of important dates here.