This sometimes starts an argument with people, but yes, roms/isos are piracy in most situations. There are various laws (both local and international) and user agreements concerning things that might affect this if you own a physical copy, the rights holder no longer sells it, the age of the IP, and so on. A lot of people get super bent out of shape when you call it piracy, but someone typically still owns the IP even if they aren't actively distributing it.

There are also sections of the Twitch TOS concerning pirated games.

IP holders may also have user agreements concerning the alteration of their software and games.

Now, with that said, the vast majority of the Retro category on Twitch (and people who stream Retro games to their individual categories) are emulating and do not own copies of the games they are playing.

Also, the vast majority of IP holders, especially for games that haven't been published in years, aren't putting any effort into going after people who download roms of these old games. This does not mean that someday some company wont go after people, though(they would more likely go after distributors). Incredibly unlikely, but they have the right to do it in most cases if they ever choose to.

You are more than likely 99.999999999999999999% safe to stream it.