Windows 10 version 1903, codenamed "19H1," was the first major feature update of 2019, released to the public in May 2019. Its ISO image is no longer officially supported by Microsoft but remains available through third-party archives. It introduced key changes such as a lighter theme, Windows Sandbox, and reserved storage.
If you are stuck with a 1903 ISO file because you have an older PC that cannot update, you should attempt to download the latest cumulative update (LCU) for 1903 offline immediately after install to secure it as best as possible, but the better solution is to use a newer Linux distribution if the hardware cannot handle modern Windows 10. windows 10 version 1903 iso