According to the latest update from Valve, not only will the Steam Deck ship on time ("Global pandemic, supply issues, and shipping issues notwithstanding, it looks like we'll be able to start getting these out the door by the end of February"), but plenty of game developers will have their own test Decks in advance to help insure their games are fully compatible with the handheld device.
Valve's Deck Verified program will give a tick to games that satisfy its categories for Steam Deck compatibility, including full controller support, as well as supporting the default resolutions of 1280x800 or 1280x700 with readable text. A look at Portal 2 in SteamDB shows that it's already set up for a Steam Deck compatibility rating, which will presumably be visible in the storefront soon.
"It's also important that we give developers the ability to test their games in order to get that nice green Verified check," Valve says, "so we've been sending developer kits out in quantity. We've been approving another wave of dev kits, and hundreds have been shipped out in the last month (and we're continuing to approve and send out even more)."
While initially planned for a launch in late 2021, thanks to hardware shortages Valve wasn't able to source all the parts needed, resulting in a delay. If you pre-ordered one, you can check your estimated reservation availability window online.