$ sed --versionsed (GNU sed) 4.4
"Whenever the pattern space is written to standard output or a named file, sed shall immediately follow it with a <newline>."
While GNU Sed's default behavior of preserving the trailing-newline status of the input's last line is defensible and can be helpful,
it should exhibit POSIX-compliant behavior when invoked with --posix.
# Acceptable default behavior - the no-trailing-newline status of the input is preserved.
$ printf 'a' | sed '' | od -t x1
0000000 61
# SHOULD include a trailing newline, per POSIX, but currently doesn't.
$ printf 'a' | sed --posix '' | od -t x1