Does anyone remember how to manually free C# managed objects?

I think I know what you mean...
I've ran into some classes with a private/protected Dispose method, but that did implement IDisposable, like the TcpClient you mention.
In these cases you have two options, cast to IDisposable and call Dispose or, the often intended method, call an alternative method, usually Close, instead.
This is probably by design and not a bug.
Probably because it makes more sense to "close" a connection than to dispose it.
I see that as far as the TcpClient goes, Dispose is accessible in .NET 4.7.2, but not 4 and 4.5 (and probably any older versions).

Another method of properly disposing these objects is by wrapping them in a using block, which still works.

var client = new TcpClient();
client.Dispose(); // Error in .NET 4.5 and ealier
client.Close(); // This works and calls Dispose() internally

using (var client2 = new TcpClient())
{
    // Use client2 here, it is automatically disposed when it goes out of scope.
    // Works even in .NET 4.5 and earlier.
}

TcpClient (i.e. "Sockets") originated circa 1983. At that time, there was no .NET and no IDisposable.

You used Close() then, and you use Close() now to close the connections and FREE resources.

TCP connections are typically "long-term" (minutes and hours and days). You maintain a long-term collection or reference. The times you can actually employ a "dispose / using" pattern in a "real situation" are slim to none.

Exposing .Dispose() is mostly redundant in the presence of .Close().