Deprecated core themes cannot be disabled on the "Appearance" tab because they are hidden, please follow the steps below.

 Stable

Step1: open stable.info.yml file

Step2: remove  below lines

hidden: true  
lifecycle: deprecated  
lifecycle_link: https://www.drupal.org/project/drupal/issues/3308890

Step3: Go to Appreance   
Now you can see uninstall option infront of your theme.

Step4: uninstall it and clear cache.

Step5: Congrats

Classy

Step1: open classy.info.yml file

Step2: remove  below lines

lifecycle: deprecated  
lifecycle_link: https://drupal.org/node/3223395#s-classy

Step3: Go to Appreance   
Now you can see uninstall option infront of your theme.

Step4: uninstall it and clear cache.

Step5: Congrats