In the new extension list, there is an X button on the right side to delete extensions. If you click this, the extension is instantly and irreversibly deleted without warning. This is a useless and dangerous feature. There already is a delete button on the top, which requires you to first select an extension and then press delete, negating accidental clicks. 3CX should remove the X next to every extension, and I also think it would be a good idea to add a confirmation ("Are you sure you want to delete these extensions?") to the regular delete button.