[philo23]

It's worth noting on top of what others have said, Gmail (and assume most others too) will only display the unsubscribe button from whitelisted mail servers, as far as I'm aware.

The reasoning being that it could be used by spammers to confirm an email address is real/valid after a user attempts to unsubscribe at which point they could sign them up to more spam. So don't expect your own emails to show the button by just adding the List-Unsubscribe header, unless you're using something like Amazon SES.

[arbitrage]

This has been oft-repeated conventional wisdom from years, but I think we're giving most spammers way too much credit.

I run several mail servers, and addresses that have not existed for well over a decade, and always give 5XX responses, still get spam.

As long as spam is profitable, because it pushes all negative externalities off to someone else, it is not worth a spammer's time to cull their lists.

I simply do not see this happening.