What online feature did Tekken 8 add for the first time in franchise history? Crossplay is the answer.
Tekken 8 introduced crossplay, allowing players on PlayStation 5, Windows PC, and Xbox Series X/S to fight one another online. This was the first time the Tekken franchise connected its major platform communities in this way.
Crossplay is especially significant for a competitive fighting game because a larger shared player pool can improve matchmaking and make online competition more active across regions and platforms. Players can also choose whether to enable or disable cross-platform play in the game's settings.
The other choices are not the canonical answer. Ranked matches existed in earlier Tekken games, and rollback netcode is an online-technology approach rather than the platform-linking feature described here. Spectator and viewing functions have also appeared in fighting games before Tekken 8, so they do not represent this franchise milestone.