They
- Deleted projects still exist in the system and are available to search results.
- Cases are also still reflected as being in that Project/Area/Milestone when you look at them. It's only that you don't see
- If an existing saved filter includes the deleted project, it does not change when the project is deleted and works as expected.
- Deleted projects are removed from all drop-down menus: for case creation and editing (except for example#2 above) in the dropdowns on case edits, and making new filters or when you create editing filters (except for #3 above).