For those interested in using this for an app on Steamdeck, you'll need to set up a custom control bind in order for it to work, and then set it as the default for your app.
Note that doing this disengages mouse input for your game from the touch screen (though the trackpad will still control the mouse), so you might want to modify your code to treat touch 0 as mouse input.
So to configure multi touch on steamdeck:
While your app is open, and you're in big picture mode (the default mode, not "exit to desktop" mode), click the physical "Steam" button on the left of the steamdeck.
Then go: Controller settings > edit layout > action sets > default > press cog > add always-on command > add command > system > touchscreen native support.
Source - Found by googling "Enable multi touch on steamdeck", in case you need other resources too.
Multi touch should work for just you now. But you'll want to make this work for everyone that launches your app.
So to do that, you have to publish the control scheme to the community. Note the ID, then go into:
https://partner.steamgames.com/apps/controller/YOUR_APP_ID
and set your control scheme as the default.
If you have listed your app as "software" not a "game" on Steam, you have to contact support to enable this option. Otherwise the tab will just be blank for you in Steamworks.
It should appear under App admin > Edit steamworks settings > Application > Steam input, if it's not there you need to contact support.