.net has had this feature for a decade++

Are you sure? All the discussion I can find online makes it seem to me like TPL and friends are just executing tasks on thread pools until completion. (see e.g., https://github.com/dotnet/runtime/issues/50796 for some discussion)

What is the virtual thread feature in .NET called? I can't find any information about it.

I don't think this is the same thing. As far as I can tell, the task abstraction is a threapool where you can submit operations and return futures. If a task blocks indefinitely, the underlying threadpool OS worker thread will be blocked, and the threadpool either has to run with less resources or spawn a new worker. Virtual threads are an M:N abstraction: blocking on a virtual thread will not block the underlying OS thread.

.NET might indeed have a virtual thread abstraction and if it does you could of course implement the Task abstraction on top of either virtual threads or OS threads, but what you linked to is not a proof that it does.

That looks similar to Java FutureTasks + Executors which is a very different concept from virtual threads.

Virtual threads mean that a blocking thread can yield to any other non blocking thread seamlessly and with very little overhead. .NET Tasks cannot do this as far as I can tell.

Yeah, so has a lot of languages. See the previous comment.

you’re trying to support “judicious and conservative” but realistically java is on a lifeline like fortran, delphi and objectiveC

That “lifeline” is apparently the same the internet is on.

What are you basing this sentiment on?

Fair enough.