LINQ for IAsyncEnumerable is supported by System.Linq.Async which is part of the reactive extensions for .NET. The reactive extensions as a whole are split into two larger NuGet packages: System.Reactive and System.Interactive.
While all the packages stayed the same, the extensions now live in the System.Linq namespace, not System.Linq.Async anymore (thanks Dzmitry Lahoda).
Relevant GitHub issue