I thought all the works from this author is in the same universe?
I mean, kaifuku jutsushi, maosama machizukuri, sono ossan and elf tensei(Novel) live close to each other. I think in the novel, revenge healer thought about going to the elves since they are fighting the royalty but decided against it.
And yes, the elves in this universe are enslaved. If you saw the novel Elf Tensei Kara no Cheat Kenkoku-ki. The elves were put in these worker camps where they live out their lives and harvested every few years. Yes harvested. The kingdom harvest them for their cores, which powers their appliances.
This isn't much of a spoiler since you will know about this in the first few chapters.