4A Games tweets that it has released the Exodus SDK, opening up Metro Exodus to modification. This page has the links to the download and documentation. The thrust of the EULA is: "you can’t use it for commercial purposes, share it behind a paywall, or sell anything you make with it." Here's more:

We’re thrilled to pull back the curtain and release Exodus SDK for you all to play with for free! To run it, you just need to have Metro Exodus (2019) or Metro Exodus Enhanced Edition installed on your machine. This isn’t just simple mod support – we’re giving you our full Editor as it was the day we released Metro Exodus, with the ability to create standalone content run from a basic executable. We’ve integrated Mod.io support, so it’s really easy to manage and share your content. If you also know visual script, our Visual Script Editor leaves the doors wide open with possibilities, so get creative! Along with some tutorial levels and as an example of a fully featured, shipped level, we’ve included a full unpacked Metro Exodus sandbox level for you to tear apart using our tools. Feel yourself a 4A Games team member – we’ve provided you with everything our Designers, Artists, and Animators have access to!