Deleted users in Process manager are still present in Workflow Accelerator.
You have to force the synchronization of users manually in Process Manager (Setup -> Manage approval workflows -> "Synchronize users now"), then the old users should be removed.
Note: In the Workflow Accelerator organization settings, you also can specify a replacement who will take over open tasks, you will find more details here: https://docs.signavio.com/userguide/workflow/en/settings.html#replacements
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