MineOS in 2018!

Is it relevant in 2018? What is her level of security and stability?

It’s Debian Jessie. If you find Debian Jessie to be secure, then that is precisely what you’re getting.

That, minus any security implications of running a public-facing nodejs server (the webui), ssh, and Minecraft instances. But generally these are tradeoffs people are willing to take in order to have multiplayer games.