Yet Another Zombie Survivors Boss Rush Unlock

Official unlock route

The official 1.0 announcement says Boss Rush unlocks after Hardcore mode for each map. The useful implication is that the mode is a later progression target, not a first-hour menu shortcut.

Checklist before you assume it is unlocked

1. Confirm the map list you are using.
2. Confirm that the relevant completion was in Hardcore mode.
3. Check whether every map has been completed under that condition.
4. Reopen the mode menu after returning to the hub or main screen.

If your menu does not match this route, record the game version and map/mode state before relying on an older guide. The announcement provides the gate, but not every UI transition.