This is about the update which launched on Steam on Oct 15 about the same time as IoW DLC was available. I haven't bought the IoW DLC and no DLCs have been purchased today so it wasn't anything like that. From monitoring my PC during the update process (which took ages!) it appears this patch required 40GB of swap files (?) in order to work. My HDD space went from 98GB to 59GB while the patching was in process then back again. The download was only 277MB. I would prefer a larger download and smaller swap files please! Other people with 22GB of spare space are reporting on Steam forums they couldn't even do the update because they didn't have enough space available.
The space reappeared after the update finished and to confirm no new preserved content came across for me today.
I think its the failed patch from the 29th September for a number of things where console got the fix, but PC did not.