I've gone through a "successful" email unsubscribe process quite a few times with some companies, only to find out that nothing really happened (keep receiving weekly/monthly spam).
These aren't shady companies either: they are large legit entities in the US.
I've given them the benefit of the doubt a few times, thinking that perhaps a bug failed to write my update to their subscription database, but after a handful of tries with the same outcome, I realized it is not isolated.
Of course I can add an email filter, but that will not take care of the root cause that most likely affects everyone else too, whether it's a deliberate attempt at denying my request to unsubscribe or simply a bug that nobody has a way to report.
What is a proper gentle nudge to say "hey, your unsubscribe system is broken, please fix it"?
There is usually no contact info available to have this kind of "out of band" communication.
Report it to their email service provider via the abuse@ alias[1]. Tell the ESP what the problem is and ask them to contact their customer and clue them in. Reputable ESPs do this regularly.
[1]: Use the “Received” SMTP headers to see which ESP a message was sent through: https://mxtoolbox.com/EmailHeaders.aspx, https://www.lifewire.com/email-headers-spam-1166360, https://www.smtp2go.com/blog/effectively-report-spam/