Hi! It can now be confirmed that Akihabara - Feel the Rhythm Remixed fully supports the Steam Deck.
Controls are as follows:
Menu
Navigate with the D-Pad
Select menu items with the Start or B button
Pause the game with the Start button. Select items in the pause menu with Start
In-Game
Move the dropping icons with the D-Pad
Switch the icons with the A button
Tap the beat marker with the B button
Activate the super charge with the X button