Currently the user needs to have a fonts replacment file in /.config/conf.d/ if they want to sync their Plasma font with SDDM.
That's not a good solution because users simply don't have this file or wouldn't know how to create it themselves.
In conjuction with https://github.com/sddm/sddm/pull/1191 this patch removes the need for additional user effort and allows for easy font syncing by writing the Plasma font value to the SDDM config file.