Even if you signed up for the mailing list in the first place? It's only spam if you never signed up for it in the first instance. If you sign up and then want to leave the list because the emails aren't relevant any more or too regular then that's just bad email marketing, not spam.
I have never signed up for a mailing list[1], e.g. gone to a "sign up for my newsletter" page and clicked "I would like to subscribe to your newsletter".
I've been signed up for them implicitly after making a purchase, etc., but I consider this spam and flag it as such.
Fair enough, that is spam in that case. Your initial comment didn't specify that you have never signed up for a mailing list (except for open source software, which is a mailing list) which I think puts you in the minority of people.
OSS mailing lists shouldn't be considered the same thing because I can send mail to them, and anyone else can as well. They are discussion forums carried out via email.
Mailing lists in the Mailchimp sense are just unwanted advertising in what I view as a communication medium - the equivalent of IRS scammer robocalls.