/usr/share/audacity/plug-ins/crossfadein.ny is in audacity-data 2.0.5-1ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | ;nyquist plug-in
;version 2
;type process
;categories "http://lv2plug.in/ns/lv2core#MixerPlugin"
;name "Cross Fade In"
;action "Cross-Fading In..."
(mult s (control-srate-abs *sound-srate* (s-sqrt (pwlv 0 1 1))))
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