Why is this not possible? Running composer install --prefer-source should get your packages as git repositories. From there you can commit, push, pull, just like normal. I’ve worked on a couple projects like that where the plugin and/or theme are separate git repo’s.
I don’t think anyone that works on Trellis is opposed to adding helpful features, but there needs to be a compelling reason. So what’s the advantage of using submodules over Composer?
I don’t know if “not built in” is the same as “demanding you abandon”.
Again, I’m not sure I follow the reasoning. Are you suggesting you have Trellis in a repository and your site in another repo? I don’t think this is impossible if you wanted to set up your project that way…
I’m interested in hearing the reason that git submodules are a feature that’s in demand.