As part of 2021 Wave 1 updates, a new feature will be added to allow System Administrators and System Customizers to configure if they want to allow a Resolved or Cancelled case to be updated using APIs, Flows or Plugins. This will be a very handy feature as we could avoid unnecessary opening and closing Case records to update fields on the Case record. The release plan does not mention classic workflow, but I do not see why we wont be able to perform the update using classic workflow / action if the record is unlocked at platform level. Please note the end user behaviours will still be the same, a read only Case record (Resolved / Cancelled) cannot be updated from UI.
Public Preview – Not Announced | Early Access – Not Announced | General Availability – April 2021
“Administrators and customizers can now update case information by using Power Automate flows, APIs, or a plug-in for cases that are in the resolved or cancelled status. The user experience on the case form will continue to be read-only.”