The main difference is about "What's a negatable option?"

(1) A single option with two behaviors (yes/no)
(2) Two different options (yes, no) but defined as a single one for convenience

In case (1), we need a tri-state to differentiate these cases:

$useAnsi = $input->getOption('ansi');
if (true === $useAnsi) {
    // force ANSI usage
} elseif (false === $useAnsi) {
    // don't use ANSI
} elseif (null === $useAnsi) {
    // user didn't pass any ANSI preference;
    // we must decide what to do
}

In case (2), you only need two cases (the third one happens automatically):

$useAnsi = $input->getOption('ansi');
$dontUseAnsi = $input->getOption('no-ansi');
if ($useAnsi) {
    // force ANSI usage
} elseif ($dontUseAnsi) {
    // don't use ANSI
} else {
    // user didn't pass any ANSI preference;
    // we must decide what to do
}

The BC break has been solved by setting the default to false for OUR --ansi option.
In that way:

• using getOption('ansi') will still return false
• people can use InputOption::VALUE_NEGATABLE with a default null value and leverage the 3 states of the option.

Thank you @jderusse.