The plugin must manage state data that is persisted between IDE executions. This means adding a @State annotation to the cached data, but more than that, it means being careful about where the state is stored, and what state we store.
Each VCS root in each project has a corresponding ClientConfigRoot, which are stored in the ProjectConfigRegistry component (specifically, the ProjectConfigRegistryImpl implementation).
However, the ClientConfigRoot only has meaning for properly configured clients. The low-level state backing these clients is persisted to the user's workspace.xml file by the PersistentRootConfigComponent class, which stores that as a list of ConfigPart instances. The user manages these through the P4VcsRootConfigurable GUI component.
If a VCS root is removed, then the corresponding list of ConfigPart must be removed, and the ClientConfig deregistered.
When a ClientConfigRoot syncs with the server, and the corresponding Client workspace has changed, then the ClientConfigRoot will need to reverify itself, and the related cache must be cleared.