Using a lot of the privacy and security measures suggested in https://www.privacytools.io/ I sometimes do a gmail search using 'is:unread in:all unsubscribe' and click on most of the links I can find.

In some cases, Gmail automatically detects the unsubscribe link. If you click Gmail's unsubscribe link, it may give you a pop-up or it may automatically write an email to [email protected]. The pop-up usually leads to a form that you have to fill to 'update your newsletter preferences'.