I was considering the ending for Honest Hearts, and when I told him to go out and explore the "Civilized Lands", it says that he was not heard from again.

Does this mean he was killed?

I'm pretty sure its just one of those things that is up to your discretion. It may have meant he died, or maybe he met found a new settlement to live in, or met a new tribe, or anything I suppose.

It could mean anything. I think he lived a good long while and was able to see many of the sites he has heard aboot. But he isn't very skilled in my opinion. I think the wasteland ended up taking his life. Whether it is in the form of raiders or mutants or the land itself is not clear. But I'm pretty sure he died. It would surprise me if he ended up dying of old age.

I like to imagine that, after a tour of the NCR core regions, he went east, eventually reaching DC. I know there's not a shred of evidence to suggest this, it just gives me a nice warm feeling to think so.

He probably walked up to the Strip North Gate, didn't understand what a "passport" was, and was then gunned down by Securitrons when he passed them in an attempt to make it to the gate.

Or he wandered into a Legion camp, talked about Joshua Graham, and was then crucified or maybe even burned on a pile of tires for mentioning the Burned Man.

Maybe he was hanging out at the Long 15 when the Courier nuked it. That would be ironic.
"Yeah, go explore the civilized lands, they're great!"
"Whoops, looks like I hit you with the weapon that I was using to break the back of civilization, sorry!!"

/stomping on optimism's face