Poorly / not at all. I just posted a steam review and emailed the dev asking if there will be functional controller support / deck support in the future.

thanks

ok, news! the dev ordered a steam deck and will be adding support.

I know my response is a year out of date but I managed to play on the Deck relatively successfully.

The graphics are fine and the button layout can be customised, so with a bit of tinkering the controls are not a problem. The only issue I had is that compared to other similarly sized games on the deck, TTL runs pretty hot and loud and the battery life at full charge was less than 4 hours while playing. I don't know enough about the technology to say what causes this, but my best guess would be there is a lot going on under the hood and it is pushing the Deck's performance.