Setting project to Wikimedia-Site-requests, as this request is about settings / configuration of a Wikimedia website.
There is nothing to change in the code base of the ULS software.
If you would like to write a software patch yourself, you will need to edit wmgULSWebfontsEnabled in https://phabricator.wikimedia.org/source/mediawiki-config/browse/master/wmf-config/InitialiseSettings.php and propose the patch in Gerrit.
Enabling this in Wikisource seems ok, The webfonts feature when enabled by default in wikipedia, will cause large performance issues and this is the reason it is not enabled and left as a preference for user(See https://www.mediawiki.org/wiki/Universal_Language_Selector/WebFonts).
I don't think T156843: Adding fonts for Church Slavonic language in ULS is about wikipedia either. Correct me if I am wrong