Comment by dataflow

It's still relevant if you use -a, -o, or parentheses (which I guess is reason to avoid them):

  $ a=1; b=1; c='!'; d='!'
  $ [ "$a" = "$b" -a "$c" = "$d" ]
  -bash: [: too many arguments
  $ [ '!' '(' "$c" = "$d" ')' ]
  -bash: [: `)' expected, found !
  $ echo "${BASH_VERSION}"
  5.1.4(1)-release

Note that it's not even a bug here; it's necessary for disambiguation.