Pushing git repo (with submodules) to google app engine

I have a project structure based on Laravel, versioned in Git, with various Git submodules (implemented using Composer) that I’d like to deploy/push to Google App Engine via Git. I’ve searched the internet for days and have tried countless tricks with recursive submodule init/updates, etc, but however I’m not having any luck.

Submodules are stuck as “references” but don’t push remotely.

Is this honestly not possible?

I’m aware of previous posts / questions (circa 2012) that mention you can’t deploy submodules to Google App Engine, but I was wondering if that hasn’t changed or if someone hasn’t figured out a workaround?

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