Hi there,
AFAIK, that was the old way now, the Build adds it as a kconfig
and there is an option for the Double tap also.
MCUBoot does support double-tap reset detection as a trigger to enter bootloader mode, but it’s optional and hardware/board-specific. NRF_SDK
In the prj.conf or via menuconfig, look for:
kconfig
CONFIG_BOOT_SERIAL_DETECT_DOUBLE_RESET=y
This enables double-reset detection when using serial recovery mode, and optionally:
kconfig
CONFIG_BOOT_SERIAL_DETECT_PORT=<GPIO_PIN>
This sets the GPIO pin used to detect the reset via an external circuit — typically a capacitor between the pin and VCC.
Also FYI, it’s present on the Nordic BLE dongle * thingy devices as well. 
HTH
GL 
PJ 
I just recieved a fresh PLUS version of the Nrf52840 & S3 so I’ll be testing with those too.