This will not be a popular answer but Linkedin and others are effectively bots so the closest thing to painful would be that the end users never see it. That means adding regex/wildcard filters for subjects and keywords in the body. Adding anything to slow down their connections will likely just trigger an error on the account saying the email is invalid and could lead to account lock-outs.

Even better if there was a shared github repo and associated addon people could use to filter these emails but there are so many different mail clients that it would become quite a project. Probably just a shared list of regex/wildcard patterns would suffice. This concept would have to become popular for them to notice their engagement emails are not increasing revenue. Spam pays. Until spam does not pay this will be a pain for all of us.

I wonder if there’s variation between EU and US for this. As a counterpoint, I’ve disabled all notifications when I set up my LinkedIn back in 2017 and to this day I receive no email from them.

But if all your notifications are off and they are still spamming, just set up an email rule to trash all the messages from their domain and call it a day. If you want to deter this behavior find the email address of someone whose time is expensive (or an alias like legal@) and adjust your rule to forward all the spam there.

Fwiw, I see it on the page as well: SuchAndSuch finsihed Zip (a quick brain teaser)... I always think: Do these people know LI is spraying the news that they're wasting time on this bs to their whole network?

What would be "painful" to LinkedIn? You can use the unsubscribe link, or call a lawyer, but he'd probably tell you to try the unsubscribe link.