Microsoft is trying to perform a smooth transition toward Universal Windows Platform (UWP) apps but the traditional desktop apps still have a long life ahead of them. For this reason, two different Skype apps exist.
One of the classic win32 desktop app that we need to manually download and install. The other is the Skype UWP app and it has a different behavior. For this reason, there are two methods to stop Skype from automatically signing in on Windows 10, depending on which app you’re using.
How to stop Skype from automatically signing in on Windows 10
As I already mentioned, the two Skype apps behave in a different manner even though both have the same features communication-wise.
Skype desktop (win32) app
The Skype desktop app’s default behavior is to keep you signed in. Of course, you can always close the app, or quit it. However, you can stop the app from automatically launching when Windows starts.
Open Skype and navigate to Tools -> Options. 
The General Settings tab has two options called “Start Skype when I start Windows” and “Sign me in when Skype starts”. If you disable both, Skype will no longer auto-start at system boot, or sign you in automatically.
You can also disable just the second option, which means you will need to manually sign in, every time you open Skype. Turning off only the first option ensures Skype no longer launches automatically when Windows starts, but you will still be automatically signed in whenever you open Skype.
Skype UWP app
I’m sure most of you won’t appreciate the behavior of the Skype UWP app, compared to its win32 counterpart. This app will automatically start with Windows and continue to run in the background at all times. Sadly, there is no way to disable this behavior other than by signing out.
To sign out of the Skype UWP app, open it and click your profile icon in the top left corner of the screen.
A pop-up menu will appear. Scroll down through the list of options until you find the one called “Sign out”. Once you click it, you’ll be signed out of Skype. Nevertheless, the app will still automatically start at system boot as there is no option to disable this behavior.
If you don’t want a Skype app that will always run when Windows 10 starts, the simplest solution is to use the Skype desktop app. You can uninstall the Skype UWP app from Settings -> Apps -> Apps & features. Then, you just need to visit the Skype website to download and install the Skype win32 app.
