The udev rule is unnecessary.
I did the whole thing again in Debian 12… without the udev rule…
just installing libusb-0.1-4… and xsane works.
In Void Linux, the situation was slightly different
$ ldd /usr/lib64/sane/libsane-brother2.so.1.0.7
linux-vdso.so.1 (0x00007fff257ad000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007faa3a090000)
libnsl.so.1 => /usr/lib/libnsl.so.1 (0x00007faa3a077000)
libusb-0.1.so.4 => /usr/lib/libusb-0.1.so.4 (0x00007faa3a070000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007faa39e23000)
libdl.so.2 => /usr/lib/libdl.so.2 (0x00007faa3a06b000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007faa39a00000)
/usr/lib64/ld-linux-x86-64.so.2 (0x00007faa3a0a8000)
So Void has the libusb-0.1.so.4
library
$ ls -l libusb*
lrwxrwxrwx 1 root root 19 Jan 7 2023 libusb-0.1.so.4 -> libusb-0.1.so.4.4.4
-rwxr-xr-x 1 root root 22432 Jan 7 2023 libusb-0.1.so.4.4.4
-rw-r--r-- 1 root root 220218 Apr 13 2022 libusb-1.0.a
lrwxrwxrwx 1 root root 19 Apr 13 2022 libusb-1.0.so -> libusb-1.0.so.0.3.0
lrwxrwxrwx 1 root root 19 Apr 13 2022 libusb-1.0.so.0 -> libusb-1.0.so.0.3.0
-rwxr-xr-x 1 root root 120912 Apr 13 2022 libusb-1.0.so.0.3.0
-rw-r--r-- 1 root root 26322 Jan 7 2023 libusb.a
lrwxrwxrwx 1 root root 19 Jan 7 2023 libusb.so -> libusb-0.1.so.4.4.4
it is a link to libusb-0.1.so.4.4.4
but the package manager does not list it
$ xbps-query -Rs libusb
[*] libgusb-0.4.8_1 GLib wrapper around libusb1
[*] libgusb-devel-0.4.8_1 GLib wrapper around libusb1 - developm...
[*] libusb-1.0.26_1 Library which allows userspace access ...
[*] libusb-compat-0.1.8_1 libusb-0.1 compatibility library
[*] libusb-compat-devel-0.1.8_1 libusb-0.1 compatibility library - dev...
[*] libusb-devel-1.0.26_1 Library which allows userspace access .
.....
So it is an old package that has been removed from the Void repo.
No new install of Void would have it.
I had better archive that file carefully now, to future-proof scanning in Void.
I found this comment
“libusb-0.1-4 is in many ways a totally different library, and you can
keep that package installed without any conflict with a newer
libusb-1.* package.”
So it is OK to keep both versions of this library.