Public transport is one of the small details that makes a city feel real. Whether GTA 6 commits to a working system is open.

What GTA games have done before

GTA 4 had taxis as a real feature - hailing, riding, paying. GTA V had a metro but it was largely scenery. RDR2 had stagecoaches and trains as proper systems.

GTA 6 has a contemporary city - buses, possibly trams in tourist zones, taxi/ride-hailing. Whether they're functional or scenery is the question.