From the beginning of development Steam Deck has been the testing device of choice, and so it only makes sense to do everything to ensure that the game plays well on the Steam Deck both from a control and performance standpoint.
Steam Deck, Xbox, Playstation DualSense and DualShock, Keyboard, and Keyboard-emulated Arcade controller sticks (Specifically the X-Arcade Tankstick) are already tested and working in Cosmic Destroyer.
In addition to other major controller support, Steam Deck requirements are something that is being taken seriously to make sure all the controls work seamlessly. Switching between tapping on the screen or using the joystick buttons or touchpads should just work out of the box in Cosmic Destroyer. The game has it's own control remapping features for the major actions, and these are largely for button mappings and to support arcade stick controllers which operate through keybindings (X-Arcade sticks, etc.). If you need to use Steam Input you can, but you likely won't have to unless you have a non-standard controller (Xbox, Playstation DualShock and DualSense have been tested to work via USB and Bluetooth). If you want to bear-claw between two devices this should mostly work, but efforts have been focused on ensuring things like keyboard+mouse work this way, and Steam Deck controls work when switching between touch screen and joystick.
As the game has no text input required, other than the score screen which uses the classic arcade input entry - there has been no need to use an on-screen keyboard prompt.