Add a Romhack tag
Modded exists but almost entirely gets you newer PC games (and the occasional chat with a moderator?). That's not what I'm looking for.
Is it really, though? Most PC games aren't distributed with the expectation of end-user modification any more than Super Metroid or Streets of Rage.
In any case, it's not Twitch's job to be the judge and jury on whether the many, many people using emulation are doing so legally or not. They can't, actually. Which is why they don't.
Also note that there's already a randomizer tag, which in the vast majority of cases is a romhack, so it's not like there's zero precedent.
Emulation is often a grey area. I'm sure Twitch won't want to promote that. Modding a PC game is vastly different from emulation.