It is one of the newest macOS versions that still supports 32-bit legacy applications.
A: You can install security updates and minor point updates (e.g., from 10.13.0 to 10.13.6) through the App Store. Upgrading to a completely new macOS version (like Mojave or Catalina) is not recommended inside a virtual machine because drivers often break. mac os high sierra vmdk download best
Though primarily for classic Mac OS (9, 10.4, etc.), both sites host community-made High Sierra VMDKs. It is one of the newest macOS versions