From my commit message:
We originally started using symfony/process because kzykhys/pygments depended upon it. But that library was unmaintained and became broken, so we stopped using it, and just used symfony/process directly. Today the only reason in favor of symfony/process is that it can pass stdin to pygments, while Shell\Command can't (T182463). But there are downsides, like not respecting the default MediaWiki shell limits, cannot be used with core's firejail support, and requiring an external composer dependency.