The version (4.0.0) of AppForSharePointOnlineWebToolkit, AppForSharePoint19WebToolkit, AppForSharePoint16WebToolkit, and AppForSharePointWebToolkit included in Visual Studio 2019 template for SharePoint provider hosted apps are all missing from NuGet gallery.
Also there are no available version of AppForSharePoint19WebToolkit on NuGet.
Create a new SharePoint Provider hosted app in Visual Studio 2019. Create a project for each minimum version of SharePoint (2013, 2016, 2019, and Online).
The included version of AppForSharePointOnlineWebToolkit, AppForSharePoint19WebToolkit, AppForSharePoint16WebToolkit, and AppForSharePointWebToolkit is version 4.0.0.
Browse NuGet Gallery for version 4.0.0 of AppForSharePointOnlineWebToolkit, AppForSharePoint19WebToolkit, AppForSharePoint16WebToolkit, and AppForSharePointWebToolkit.
None of the versions are available in the gallery.
The versions included in the Visual Studio Template should be available on NuGet. For AppForSharePoint19WebToolkit there are no versions available.
Visual Studio 2019 (latest official version)
It is impossible to use source control and build servers as restore NuGet packages will fail because the packages can't be found on NuGet.
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Definitely an issue. Moved dev environments today and hit this problem. Still had previous environment available so simply copied the package over manually.
Confirmed... I've raised it with engineering...
For ref, here's what a default package.config contains:
<?xml version="1.0" encoding="utf-8"?>
<packages>
<package id="Antlr" version="3.5.0.2" targetFramework="net472" />
<package id="AppForSharePointOnlineWebToolkit" version="4.0.0" targetFramework="net472" />
...
</packages>
While the NuGet repo isn't current: https://www.nuget.org/packages/AppForSharePointOnlineWebToolkit/

I am facing the same issue. Any temporary solution?
I am wondering where the package is getting downloaded the first time we create a solution. This issue came up when another developer started working on the same solution and tried to restore the package.
It's located here: C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\Common7\IDE\Extensions\Microsoft\SharePoint\NuGet Packages\
I've brought it to engineering's attention... they are aware.
Packages aren't downloaded when you first create a solution. They are pulled from a local cache as @jensotto says, which is setup when you install the tools.
@jensotto Is there any command which we can use to download the NuGet package from the local cache(as @andrewconnell mentioned)?
For time being I am going to use below approach to download NuGet package from the local cache
https://stackoverflow.com/questions/10240029/how-do-i-install-a-nuget-package-nupkg-file-locally