As I already reported earlier this year, Windows 10 May 2019 Update will reserve 7GB of your storage capacity to prevent out-of-disk error issues with the installation of future updates. This disk space will be permanently blocked, but users will be able to manually reduce the amount of space reserved.
As ARS Technica reports, this may make it impossible to install the latest versions of Windows on systems with limited disk space:
If your system can't be upgraded, it'll be stuck with version 1809 for the remainder of its supported life; that's currently set to expire on May 12 2020 for Home, Pro, and Pro for Workstations editions, and May 11, 2021 for Enterprise and Education editions. Version 1809 also has a long-term servicing channel counterpart, which will receive bug fixes and security updates until January 9, 2024 and security fixes until January 9, 2029. It would be nice if Microsoft offered these fixes to hardware that's left abandoned by a feature update, but we wouldn't hold our breath.The new disk requirement does not apply to Windows 10 IoT Edition.