missing-root MicroG is a FOSS reimplementation of what Apps think Playservices do
The trouble with this neat statement is that the reality is more complex.
Its explained in this discussion
Croak3114 You can read some of the discussion here: https://discuss.grapheneos.org/d/4290-sandboxed-microg/11
microG downloads proprietary Google libraries and then uses them.
Also apps that communicate with Google Play or microG contain proprietary Google libraries that handle that communication. Those libraries are also set up to communicate directly from the app with Googles servers, they dont need to pass that communication to the Play apps or microG.
For example you can see here details of how firebase libraries work on devices with or without Play Services
https://firebase.google.com/docs/android/android-play-services
When compared to sandboxed Play it becomes questionable what functional security or privacy guarantees can be gained by microG being FOSS.
There are security benefits from using sandboxed Play compared to microG. There is also much better app compatibility.
Its highly unlikely GrapheneOS will ever have support for microG. Its already a lot of work maintaining sandboxed Play. Trying to also implement support for microG in a way that met GrapheneOS security standards and then to maintain that compatibility up to these standards going forward would be a load more work for questionable benefits. There is much more important work that the GrapheneOS team wants to focus upon.