Any existing current streaming software is suitable to start a Twitch Watch Party. No matter if it is Streamlabs OBS, OBS Studio, Twitch Studio or any other broadcaster software.The reason for the simple answer is the following: The function to start and run a Watch Party is not done in the software, but online via Twitch's servers. Your streaming software is only responsible for the transmission, just like any other content you show on your Twitch channel.