Completed, Superseded and Abandoned plans are history: they leave In progress but stay in the project with their whole record.
To see them, choose Completed in the filter above the list (In progress, Completed): they are grouped under headings such as Completed · 4, Superseded · 1 and Abandoned · 1. Completed is for work that is done, Superseded for a plan replaced by another one, and Abandoned for one dropped on purpose. Only an agent sets them, with project_program: the app has no status to choose, and a plan you add starts In progress.
Agents are told not to reopen archived items: an agent that says it is implementing one gets PLANNING_PHASE_MISMATCH from planning_preflight. If a need comes back, add a new plan that says why.