System settings contains a number of KCMs in the Network > Settings category.
Some of these settings are only relevant for apps using KIO while others may apply to other apps as well. It is not very clear which setting applies to which kind of app.
The usefulness of some of the settings is questionable as well, e.g. the default username/password for Samba shares.
Proxy:
It's not clear at first glance whether this configures the systemwide proxy or jsut for KIO. Since there is an option to use the system proxy configuration I assume it's the latter. It's also not clear to me what the difference between the "Auto Detect" button there and the "Detect proxy configuration atomatically" is.
Connection Preferences:
I assume it's only relevant for KIO. I'm not sure any of these settings are useful. There already was some discusson in T12285.
SSL Preferences:
Lives in kdelibs4support. Why? A UI for managing a systemwide certificate store is likely useful, think revoking a compromised certificate.
Cache:
Only affects kio-http's caching as far as I can see. Mostly useful for Konqueror. Konqueror has an additional settings thing that writes into the same settings: https://invent.kde.org/network/konqueror/-/blob/master/plugins/khtmlsettingsplugin/settingsplugin.cpp
Cookies:
Used by kio-http. Mostly relevant for Konqueror.
Browser Identification:
Probably mostly relevant for Konqueror. Or is there a use case for changing the user agent for the webdav KIO slave?
Windows Shares:
Completely useless IMO. Removal proposed in https://invent.kde.org/frameworks/kio/-/merge_requests/212
A lot of it boils down to the question whether kio-http is used outside of Konqueror.
In cases where the settings are clearly only relevant for a limited subset of apps it might make sense to exclude them from systemsettings and have them only in the apps themselves.
Another idea that came up was folding stuff like proxy and timeout settings into the connection settings.