When the Arduino is in reset mode are the digital pins high impedance?
From the ATMEGAxx8 datasheet...

11.1 Resetting the AVR
... The I/O ports of the AVR are immediately reset to their initial state
when a reset source goes active. This does not require any clock
source to be running.

(bold added)
Looking over the register descriptions for the Data Direction Registers, all bits have an initial value of 0, corresponding to high impedance input.
...so it is safe to assume that all IO pins will be in a high impedance state a very short time after the RESET pin goes low.
Caveats:

The clamping diodes are in-circuit even when pins are in high impedance state, so current will flow if the voltage on the pin is higher than Vcc or lower than Ground (at least until the diode blows up!).

Some Arduino pins are connected to other things besides just the pin on the MPU. For example, digital pin 13 is connected to ground though a resistor and diode, so current will flow into it if you connect a voltage higher than the threshold for the diode. On the Uno there are also resistors on the RX and TX pins connected to the serial communications link, so current can flow there too.

The MISO pin can inadvertently become an output while RESET is held low if the Programming Enable command ($AC $53 xx yy) is transmitted over the MOSI &amp; SCK lines. This process is described in App Note AVR910.