How do I forcefully uninstall any module having content?
For example, I made any custom content entity type and add content
  into it. Now I can not uninstall my custom module until and unless I
  delete all content or truncate module specific db table – Sugandh Khanna.


Unfortunately, in Drupal 8 you can't disable a module, like you can in Drupal 7.

In Drupal 7 you need to disable the module first, which means all module related code stays in DB. Then, if you wanted, you Uninstall to remove everything from DB. 

In Drupal 8 the only option is to uninstall the module, which means you have to destroy all traces of your module from the Database. You cannot just disable the module anymore.

Also, to uninstall Flag (as in your comment here), you have to uninstall dependencies first, then and only then, it will allow you to uninstall Flag. Because if module X depends on Flag and somehow you removed flag, it will totally screw up module X, X becomes useless, module X is nothing without Flag. Therefore you need to remove module X first, then and only then it will allow you to remove Flag.

For more info: Why can't I uninstall modules in Drupal 8? (Explained)

Issue: Disabled modules are broken beyond repair so the "disable" functionality needs to be removed