First, make sure you've rebooted. I've occasionally have wifi stop working, and have to reboot to fix it.
Second, what decky plugins do you have? Just decky itself shouldn't affect it, but if you have any plugins that modify wifi, they're naturally suspect. I would suggest uninstalling them, or temporarily moving their plugin folders out of the decky folders in desktop mode.
If you've already rebooted, and you don't have any wifi/internet related plugins, try turning on "Force WPA Supplicant" in the steam settings. I think you have to enable the developer menu under Steam System settings, and then you should find the option in the developer menu. After turning this on, try rebooting.
Finally, try swapping back to your previous Steam update partition. Power off the deck, and then hold QAM button (the ". . ." button) and press the power button once. It should open a menu where you can pick to boot which of the two system partitions to boot from (SteamOS uses two system partitions, when you install an update, it installs that to the partition you're not using, and then boots from it. That way if anything with the update breaks, you still have your previous working OS still available.). Pick the second option from this menu, which should be the pre-update OS version.
Please let me know if any of these work for you, or if you need more details on part of it.