1. GMail will block your email if you don’t allow one-click unsubscribe. But this is very insecure since anyone can unsubscribe you if you forward your email

Easy Unsubscribe: Implement easy unsubscribe options (One-click Unsubscribe). Gmail users have tools to report spam, unsubscribe from unwanted emails and control their inbox experience. If it is too difficult to unsubscribe from your emails, customers will be more likely to flag your email as spam. Additional links provided in the ‘References’ section at the end of this article.

2. At the same time, Apple’s ITP will start removing all the information from the URL and only leave the domain, if it classifies your site as a “bounce tracker”. This means you won’t even know who to unsubscribe on one click! So all your emails will be blocked.

https://getcake.com/apples-intelligent-tracking-prevention-2...