Example: Characters spend an entire story trying to break out of walled-in place, in the end, they break out of walled-in place. What happens outside is left to the reader's imagination.
Is this an acceptable thing or is it regarded as lazy?
I think it depends on whether you're doing it out of laziness. Oftentimes the reader can tell the difference. I'd recommend not leaving it open-ended if you're just trying to add drama (which is just a personal preference, so don't take it to heart), but other than that, many books end with these, so it'd probably be acceptable for you to do so, as well.
I like open ended endings as long as they kind of hint at being happy. Because it lets me imagine that all of the things that I wanted to happen in the story actually happen.
Plus cannon
I'm not a huge fan of open endings, but I guess I can kind of be ok with it if you kind of hint at what happens (sort of the same as what @Screaming Cinnamon Roll said). But I find that an open ending can really make or break a story for me.
Like I don't really see it a lot in published books but I do see it a lot in Creepypastas and r/nosleep stories where a story just kinda… ends? And it feels like it's supposed to be this awesome cliffhanger ending but it kinda falls flat. That's super frustrating because I just read this entire thing waiting for an explanation for what's happening and then I never really get one. But if you end your story and just the right moment, where you leave just enough information for the reader to piece things together. I'm pretty good with stuff like that.
So in regards to your story, I would say that if you hinted at what happens to your characters then that's good.
If you just kind go "They broke out of the walled city. The end." That's lazy.
What I'm taking from this is
1- Give the reader what was promised.
2- Make sure it isn't abrupt or random.
3- Give enough so the reader can imagine what could happen to the characters.
I was really vague earlier but lemme explain my scenario and yall can give me more specific advice
.
The two characters run away from their cult town to see the world. They meet people and explore the country and such other stuff happens but that's not important. Eventually, they learn about the wall around the country. They are repeatedly told that only death waits out there and such but they decide to leave the country. They don't break the wall, they climb over it and then they see an ocean and decide to cross it and see whatever is on the other side.
Is that too vague?