Yes, the powersupply has to be 9V - 2A with 'Center Negative' (2.1 mm) barrel .
In 2019? That is f**ing EVIL! 
Otherwise looks like a great piece of kit (kudoz for that!), but center pin negative on a DC plug is so uncommon these days, that's like begging for trouble. IDIOT design decision, no excuse.
I assume they put in reverse polarity protection to go with that? Right..?? RIGHT??
Yes, the powersupply has to be 9V - 2A with 'Center Negative' (2.1 mm) barrel .
In 2019? That is f**ing EVIL! Otherwise looks like a great piece of kit (kudoz for that!), but center pin negative on a DC plug is so uncommon these days, that's like begging for trouble. IDIOT design decision, no excuse.
I assume they put in reverse polarity protection to go with that? Right..?? RIGHT??
I generally use a standard router power supply +12V 1A center positive.
Here is an old email explaining how it works. 
From: 8bits
Sent: Wednesday, May 23, 2018 8:04 PM
To: KdL
It can take any power supply between 7 and 28v (I recommend 12v so the converter does not stress so much). There is also a polarity rectifier circuit so you dont have to worry about the PSU polarity.
The button nearest to the DC barrel connector is ON/OFF. For the button to act, you need to let it rest for a couple of seconds after each push.
8bits4ever Team
It can take any power supply between 7 and 28v (I recommend 12v so the converter does not stress so much). There is also a polarity rectifier circuit so you dont have to worry about the PSU polarity.
Ah okay, no worries then! 
In this case specs should be updated to reflect that, though.
Out of curiosity I searched, found a detailed view of the board in this thread, and had a look. I noticed a couple of things about the power circuit:
a) For polarity protection, personally I'd put in a single 1 or 2A rated Schottky diode rather than bridge rectifier. Simpler, lower losses, lower voltage drop of ~0.3V (versus 1.5~2.0V for rectifier bridge with 1N4007 or similar). And - of the DC jack can connect directly to circuit GND which which can be useful sometimes. Stupid user who puts in wrong polarity then simply has no power, no harm done.
b) All 16V rated caps there, meaning: input is only rated for 16V max? (well a bit more due to diode drop). I'd put 25V or 35V rated cap on the input. DC/DC converter circuit has no problem with higher input voltage, so higher rated cap = wider choice what people can use as power supply. Note that this also applies to the rating of ceramic cap(s) on the input.
c) Most interesting: looks like a 680 uH inductor there? That's a really high value for use with an LM2596. Side effect would probably be a low voltage ripple on output, which is nice. But it would also limit available output current. Have they tested (as in: measured) output current this circuit (as seen in photo) can supply? I'd rather expect an inductor in 33..47 uH range there. 68 uH is already a high-ish value. Let alone 10x that.