It took me a while to figure out that Slack was using slackb.com to implement its upgrade antifeature so I turned that off. I was happy for a couple months then decided it was a good time to upgrade so I re-enabled that URL, launched Slack, received the annoying dialog box, and upgraded. Great, that's how it's supposed to work in the first place! Then I re-disabled the URL.
But... Slack changed. It changed the URLs it used in the antifeature. Moreover it now uses slackdns.com "for some purpose" presumably to circumvent DNS systems implemented by users.
Let me put this in bold. Hey Slack, we use our own DNS servers to protect ourselves from you. Same reason we use VPNs and single-account email addresses. We don't trust you and we trust you even less as you circumvent our protections.
Okay enough carping, blocking these URLs worked for me. Drop these in your /usr/hosts file and Slack should stop annoying you.
# prevent Slack from constantly updating
All this effort just to control when new software installs. Ridiculous.