WWDC 2013 Session 707 "What's New in Kext Development" [1] is about kext signing and where 3rd party kexts should be placed in 10.9+. In the PDF slides you can see on slide 101:

"Get out of /System/Library/Extensions ... in fact ALL of /System (talk to us if you think you need to be here!)"

And then the major point which is only part of the spoken presentation and not IN the PDF itself:

"And another warning I'll throw out here is in the future as we start to lock down the /System folder, you might actually get write errors. So when you try to install a kernel extension into the /System folder, the write itself may fail."

That sounds like either read-only mounting of a separate partition, or kernel level write protection, or some similar solution.

If you want to read it instead of watching the video, you can visit the transcripts for this session over at asciiwwdc.com[2] and search for the phrase "lock down".

[1] https://developer.apple.com/videos/wwdc/2013/?id=707

[2] http://asciiwwdc.com/2013/sessions/707

Thanks, that is very good to know.