The ExecReload command calls kills on a process owned by the unbound user (or whatever user is configured). To do so, it needs the CAP_KILL capability.
ExecStart=@UNBOUND_SBIN_DIR@/unbound -d
NotifyAccess=main
Type=notify
-CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_BIND_SERVICE CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_SYS_RESOURCE CAP_NET_RAW
+CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_BIND_SERVICE CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_SYS_RESOURCE CAP_NET_RAW CAP_KILL
MemoryDenyWriteExecute=true
NoNewPrivileges=true
PrivateDevices=true