In short, they say they won't sell your email address or utilize that information for anything other than the operation of the email list.
You're free to unsubscribe from Some Party at any time. You're also free to avoid touching the 3rd party email service by using the Some Party RSS feed to roll your own. The contents of every newsletter will be shared there in full.
The code for Some Party is published at GitHub and available under an MIT license.
The online music ecosystem relies on third-party companies to host music and video. It's likely that any of the services I share media from (YouTube, SoundCloud, Bandcamp, Instagram, Twitter, Vimeo, etc) will include a traffic measurement mechanism in their embedded media players.
The only other third-party service used here is Google Tag Manager and Google Analytics. My feelings won't be hurt if you block those on your end, but if you do please say hi once in a while so I know you're out there.