Whether your Steam Deck uses the new Wi-Fi setup in SteamOS 3.9 depends on how you got there. According to the SteamOS 3.9.3 beta notes, as reported by MIXED, a fresh 3.9 install now defaults to wpa_supplicant, which Valve says improves compatibility with different networks. A Steam Deck or Steam Machine that was upgraded stays on the older iwd backend until you change it.
The change applies to the whole 3.9 line. Valve also improved wpa_supplicant so it reconnects quickly after sleep, something that used to work only with iwd.
The switch has helped some Steam Frame owners. On a Steam bug report, two owners said the Frame's wireless adapter wouldn't scan for networks under iwd and never tried to connect. Both fixed it by moving to wpa_supplicant. One owner could still join a normal 5GHz network under iwd and suspects the problem is the hidden 6GHz network the adapter uses. Valve hasn't responded on the thread, and its notes only mention "improved support" for the adapter.
If you installed SteamOS 3.9 from scratch, you're already on wpa_supplicant. If you upgraded, you can switch by turning on the wpa_supplicant option in Steam under Settings, then Developer. One of the Frame owners turned on "force wpa_supplicant backend" and rebooted.