From my experience with the steam deck:

If you're already logged in, you can use it online. But if it decides to log you out, and you're offline, then you're out of luck.

Older versions allowed you to switch to desktop mode without logging in (with broken on screen keyboard, since that's only available once you log into steam), but that option is missing in newer versions.

▲

koolala 2 hours ago | parent [-]

They changed its location and it was bugged for an update. You go to Settings > System and then enable Show Switch to Desktop Mode.

▲

CodesInChaos 2 hours ago | parent [-]

For me the "Switch to Desktop mode" button only showed up after logging into steam but was hidden while logged out.

	▲	koolala 2 hours ago \| parent [-]
		Right that setting is to make it show when logged out. You can get to the setting without logging in.