Hmm, it wasn’t on any of my systems… Let me check the help file. ^.^;
core.excludesfile
In addition to .gitignore (per-directory) and .git/info/exclude , Git looks into this file for patterns of files which are not meant to be tracked. “~/” is expanded to the value of $HOME and “~user/” to the specified user’s home directory. Its default value is $XDG_CONFIG_HOME/git/ignore. If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/ignore is used instead. See gitignore(5).
Ooo, I’m very sure it used to not have a default, but now apparently the default is $XDG_CONFIG_HOME/git/ignore falling back to $HOME/.config/git/ignore. 
