Nowadays I use Skype only once in a while when no other choice or people insist to see me there.
It is NOT in my Start up Applications list and in Skype settings it is ticked to NOT start automatically. But it does starts automatically each time I reboot.
I installed Skype via the Software Boutique on 19.10
After starting the application it places an Icon in the panel. After shutting down the program and right clicking on the icon and qutting that removes the application from the panel. Upon a restart the Icon does return to the panel.
Opening Control Center > Startup Application and unchecking the box next to application will prevent Skype from starting upon a reboot.
This will persist until you run Skype again and you will have to follow those above steps again.