Depending on what you call "proper modular device driver system". Almost all device drivers are built as separate binaries, so it has one if that's what you are asking for. The ones which are under GPL usually sit in the single source tree of the upstream kernel, because that makes maintenance easier.
There can be drivers under GPL which are nevertheless not submitted (or accepted) into upstream, and there can be also drivers which are not under GPL, in both cases we are obviously talking about separate projects, meaning their development would not be reflected in kernel.
It seems you are referring to number of changes in drivers in the upstream kernel to imply on the availability of the drivers outside of it. Which makes no sense to me.