All UXP-Hardened-Preferences packages also known as iceweasel-uxp-hardened-preferences and icedove-uxp-hardened-preferences are being removed as we have added custom firejail profiles for each UXP application.
The about:config options were useful prior to our transition to UXP, when we did not have control over upstream (Mozilla) sources.
As we have been implementing the features directly into our applications, these left-over hacks are no longer needed.