client/state/initial-state.js
function shouldAddSympathy() {
	// If `force-sympathy` flag is enabled, always clear persistent state.
	if ( config.isEnabled( 'force-sympathy' ) ) {
		return true;
	}

	// If `no-force-sympathy` flag is enabled, never clear persistent state.
	if ( config.isEnabled( 'no-force-sympathy' ) ) {
		return false;
	}

	// Otherwise, in development mode, clear persistent state 25% of the time.
	if ( 'development' === process.env.NODE_ENV && Math.random() < 0.25 ) {
		return true;
	}

	// Otherwise, do not clear persistent state.
	return false;
}
function shouldAddSympathy() {
	// If `force-sympathy` flag is enabled, always clear persistent state.
	if ( config.isEnabled( 'force-sympathy' ) ) {
		return true;
	}

	// If `no-force-sympathy` flag is enabled, never clear persistent state.
	if ( config.isEnabled( 'no-force-sympathy' ) ) {
		return false;
	}

	// Otherwise, in development mode, clear persistent state 25% of the time.
	if ( 'development' === process.env.NODE_ENV && Math.random() < 0.25 ) {
		return true;
	}

	// Otherwise, do not clear persistent state.
	return false;
}
Determines whether to add "sympathy" by randomly clearing out persistent
browser state and loading without it

Can be overridden on the command-line with two flags:
- ENABLE_FEATURES=force-sympathy npm start (always sympathize)
- ENABLE_FEATURES=no-force-sympathy npm start (always prevent sympathy)

If both of these flags are set, then `force-sympathy` takes precedence.

@returns {boolean} Whether to clear persistent state on page load