/lib/udev/rules.d/70-btrfs.rules is in btrfs-tools 3.12-1.
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 | SUBSYSTEM!="block", GOTO="btrfs_end"
ACTION!="add|change", GOTO="btrfs_end"
ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"
RUN+="/sbin/modprobe btrfs"
RUN+="/sbin/btrfs device scan $env{DEVNAME}"
LABEL="btrfs_end"
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