From the help file (found by searching for default spatial interpolation)
To change the default to linear interpolation, choose Edit > Preferences > General (Windows) or After Effects > Preferences > General (Mac OS), and select Default Spatial Interpolation To Linear. Changing the preference setting does not affect keyframes that already exist or new keyframes on properties for which keyframes already exist.
Also, it's in the first page of the preferences. You might have looked there first :)
- The Great Szalam
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