It won’t brick your device, however it will not be very useful with the Steam Deck.

You will only be able to charge your Steam Deck once at best, while not playing (because you will drain faster than it charges) and that’s if the battery was fully charged to start with.

You appear to be under the mistaken belief that plugging usb devices into power supplies/banks that don’t support the most desired voltage/wattage will brick the device. This is not the case. When usb devices plug in, correctly designed devices (of which the Switch, Steam Deck, and Anker banks are all examples) will negotiate with each other to determine what they support. With my Switch for example, I have plugged it into a large number of ports, and since supporting the Switch’s desired voltage is somewhat rare, nothing happens, no charging, but also definitely no bricking. The Steam Deck, on the other hand, doesn’t mind it when the device has a lower wattage, but the lower wattage does mean that it will charge slower, or possibly drain the battery slower if the wattage is lower than consumption.

The Steam Deck is actually less picky than the average post here claims. It will even charge on 5V 1A. Not quickly, of course, and doing anything at all will drain the battery faster than it charges, but it can. One way to find out is to see if the % changes up or down while plugged into the bank.

Obviously, an incorrectly designed 20 volt PD charger that doesn’t negotiate could damage devices, but such a device would have poor reviews.

Yes you can use it while playing. Depending on the game game it will keep the deck fully charged or it will slowdown the discharge. You will get a warning message about slow charger which you have click ok on.