Can users be forced to insert only plain text by default in CKEditor?
You want to change the ckeditor configuration to always paste as plain text.
This defined in https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-forcePasteAsPlainText
It comes down to the following JavaScript configuration:
// All content will be pasted as plain text. 
config.forcePasteAsPlainText = true;

Have a look here for where to set it
Is there advanced configuration for CKEditor in Drupal 8?
Or just use module CKEditor custom config and set in custom configuration: forcePasteAsPlainText = true