In an updated support document, Microsoft has stated that the 32 GB storage requirement for Windows 10 version 1903 only applies to OEMs when they manufacture new PCs. This new requirement does not apply to existing devices. It is not known what the minimum storage requirement will be for existing PCs that install or upgrade to version 1903, but could be 16 GB like the previous version. This means existing PCs, or even custom built PCs, will continue to receive updates and the 1903 update will require about the same amount of free disk space as previous updates.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-to-require-32gb-of-storage-only-on-new-oem-pcs/