Back in late April, Microsoft updated the storage requirements for the Windows 10 May 2019 Update (Version 1903). In fact, it was the first time that any changes had been made to the storage requirement for Windows 10 since it was first released back in the summer of 2015.
The May 2019 Update now asks for at least 32GB of storage to install on 32-bit and 64-bit versions of the operating system. Previously, those requirements stood at 16GB and 20GB respectively. Microsoft is now clarifying that these new, higher storage demands only apply to new devices shipped out by its OEM partners. It does not apply to existing systems that already have Windows 10 installed.
Microsoft goes on to add, "PCs that don’t meet new device disk space requirements will continue to receive updates and the 1903 update will require about the same amount of free disk space as previous updates."
In other words, if you were previously able to get by with a low-end Windows 10 PC with less than 32GB of storage, you will have no problems installing the Windows 10 May 2019 Update. It's quite possible, however, that Microsoft might cut off these sub-32GB devices in a future update as Windows 10 demands grow.
We should mention that along with Microsoft's revised guidance on Windows 10 storage, it puts a note at the end referencing Reserved Storage. Reserved Storage sets aside roughly 7GB of storage for use by the operating system. "Reserved Storage sets aside disk space to be used by updates, apps, temporary files, and system caches," Microsoft writes. "It improves the day-to-day function of your PC by ensuring critical OS functions always have access to disk space."
According to Microsoft, Reserved Storage is enabled by default on new PCs shipping with the Windows 10 May 2019 Update installed, hence the 32GB mandate for those systems.