That update, currently being tested by Windows Insiders under the code name 19H1, is expected to be publicly available in the March/April 2019 time frame.
The blog post explains the concept behind this new Reserved Storage feature and explained that its primary goal was to insure enough disk space is available on Windows 10 systems so that future updates do not fail due to lack of disk space. For many owners of small form factor devices, those that have 32GB and 64GB of initial space, the jury is still out on how well this might help those low end devices. However, if your computer has a 250GB or higher disk drive, this will likely be easy to implement and depend on for this in future upgrades.
Right now, if you want to try this out you need to be a Windows Insider and follow the steps in a quest that is posted in the Feedback Hub.
The quest requires a change in one registry entry on the device, so as always, make sure you know what you are doing when making changes to the registry otherwise, things can get messed up pretty good. I activated this registry entry on all three of my bare metal testing devices while they were on Build 18309. After installing Windows 10 (19H1) Build 18312, I now have the Reserved Storage data listed on all three devices.
As most tech sites have already highlighted, Reserved Storage will start out setting aside about 7GB of your hard drive space. As you use your system, that amount of reserved space will fluctuate to make sure enough is available for future system updates.
In my screenshot gallery below, I have several different amounts set aside for four bare metal devices and one virtual machine.
Only one of those physical devices and the one virtual machine actually set aside 7GB as a starting point. The others range from a low of 2.5GB and the last two with approximately 5.5GB set aside. Over the course of the last few days, I have seen some very small fluctuations in these numbers but not gigabyte levels of change. As noted by the storage team, this should settle in as you use the system.
While many do not like the idea of space being reserved and set aside on their devices, many enthusiasts want to exercise complete control over their systems, making this part of the base system is a smart move by Microsoft. Feature updates are a critical aspect of Windows as a Service (WaaS) and providing space for those to have a better chance of success makes a lot of sense.
Our devices have so much storage these days, having some of that reserved for the system is just fine with me in the long run.
One detail that has not been provided at this point as the feature is being tested by Windows Insiders is whether this will be an option when Windows 10 (19H1) becomes available this spring. I think enthusiasts will appreciate that as a choice, but just like automatic and mandatory updates, for everyday users this is a must have option.
Windows 10 (19H1) Reserved Storage Screenshot Gallery