To play Tanki Online via Steam Deck, you need to manually enable the «Steam Linux Runtime 3.0 (sniper)» compatibility mode in the project settings on SteamOS.
Go to the game page on your Steam Deck and click on the gear icon on the right. This will open the settings window. In the "Compatibility" section, enable forced use of compatibility settings and select "Steam Linux Runtime 3.0 (sniper)".

Another important feature: controllers on your Steam Deck emulate keyboard and mouse control. This is an experimental control option. Support for analog control for gamepads will appear later.