Some behaviors show bosses that you don't handle an employee's termination well. It is important to distinguish unprofessional behavior from the good of the company.
If you have resigned, you no longer exist for your boss. Your comments are ignored, and your boss only talks to you about the bare minimum in a calm tone. Your boss denies you freedom of choice and wants you to feel bad. You are made to feel guilty. You may also no longer be allowed to attend meetings. You can, for example, be released because you are “afraid of revealing secrets” or because there is no longer any trust. This is a common procedure in some industries. It can be the case if the boss has already finished with you. On the other hand, it is also possible that he will resent your decision. You will find out how to recognize a bad boss if you have quit your job. You should tell your boss that you are changing jobs. This can show whether you had a good or bad boss.