/lib/udev/rules.d/66-snapd-autoimport.rules is in snapd 2.32.5+18.04.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # probe for assertions, must run before udisks2
ACTION=="add", SUBSYSTEM=="block" \
RUN+="/usr/bin/unshare -m /usr/bin/snap auto-import --mount=/dev/%k"
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