I have only ever had this issue with discord on arch.
The issue you describe is not Arch specific and it's not an issue. Using a package manager means using a program to manage your packages. Things can't auto-upgrade, that breaks the point of a package manager.
Whenever discord has an update, it will not fetch the update, but it tells me that an update can be downloaded.
Of course, if you install discord through pacman, then pacman manages the update.
As for the JSON file that's a very hacky approach, discord shouldn't outright fail to launch if there is an update. And in fact the Arch wiki says it has a flag to skip the version check completely:
To disable the update check, add the line "SKIP_HOST_UPDATE": true to ~/.config/discord/settings.json. If the file does not exist, create it and add the following:
~/.config/discord/settings.json
{
"SKIP_HOST_UPDATE": true
}
More info on https://wiki.archlinux.org/title/Discord