You can insert the SpatialOS plugin inside the UE4 editor in order to simplify and give the opportunity to those who want to develop an open world multiplayer game with a large number of players, can do so by simply activating the plugin. Sorry for my english.
I’ve been working on it for a project of mine for weeks and I know it, I meant inside UE4 as an activatable plugin.
They won’t implement it as a built-in plugin I’m afraid - it’s not their plugin to distribute and judging by SpatialOS’s website you need an account with them to use it anyway.