Java 8 still gets updates, but you have to have a license to get them for business use (kind of like paying for extended support for Windows 7). And then things like OpenJDK fill the gap by making an open source release available that doesn't cost money you can drop in as a replacement.
Sure, there are still products that use it (the oldest legacy Java app I still see runs on OpenJDK 11 just fine, mind you), but if you're launching a new package manager in 2020, the only apps that will use it are new apps. So you might as well start with the latest possible release. There's no good reason for Windows Package Manager to start with OpenJDK 8 as the baseline.
