Sales and Service cloud is not marketing platform like Marketing Cloud. So, the Email Opt Out field in Salesforce works as below.

The Email Opt Out field will allow you to exclude marked contacts from mass e-mail.

Email Opt Out field shows a message/warning when the user tries to send an email.

This field doesn’t block to send email. Check in apex, workflow rule and process builder before sending email alerts.

Note:

Make use of Individual object in Salesforce to handle Email Privacy Settings.

Check the below link for more information

https://www.infallibletechie.com/2018/03/what-is-use-of-individual-object-and.html