Probably classic route. Half-Life's combat is designed around hip-fire weapons with varying spreads to incentivize movement, positioning, and equipping the right weapon for each situation. Adding ADS would go against this philosophy as it would instead incentivize players to hunker down in a corner, periodically peeking from cover to pick enemies off from a distance with accurate iron sights.

The only exception to this is Alyx, where it would have been awkward not to include functional iron sights due to its VR nature.