Reading through their mail merge tutorial, their method looks insanely risky: putting all addresses in “to” and rembering to click another button.

That’s really interesting. So does that mean the interpreter just checks whether the current line is more indented, less indented, or equal vs. the preceding, without caring by how much?

Please elaborate (eg which standard is this defined in?)

I meant it more in the sense of one channel, when shutting down for the night, emitting the “next channel” tone such that every viewer’s set would change to a channel that was still broadcasting.

Would it have been possible for the speakers of the time to emit those frequencies? Imagining the equivalent of a Twitch raid: “I’m done broadcasting so I’m going to send you to the next channel.”