Thankfully, even though the Mastodon post that triggered this made the usual mistake of implying that disabling NAT results in "exposed" LAN devices (NAT has nothing to do with it, that's the firewall's job), the author of the article does not.
What does the Steam Deck's OS use for networking? If it's NetworkManager and NM has been configured to check an IPv4-only URL for connectivity (*), that might be the problem. Though of course, since Steam services themselves don't support v6 (as the author notes later), working v6-only network on the Deck won't help with playing games anyway.
What does the Steam Deck's OS use for networking? If it's NetworkManager and NM has been configured to check an IPv4-only URL for connectivity (*), that might be the problem. Though of course, since Steam services themselves don't support v6 (as the author notes later), working v6-only network on the Deck won't help with playing games anyway.
(*): https://networkmanager.dev/docs/api/latest/NetworkManager.co... / https://wiki.archlinux.org/title/NetworkManager#Checking_con...