Signal has been indispensable software for several years now but unfortunately its development path has not been so pure as its origins. Born from an unmitigated desire to deliver secure texting which is useful to end users, it now has an inexplicable insistence on taking away from users the ability to control how often the software annoys you to update.
It updates constantly. If you tell it no you don't want to update, it will ask you again in a couple minutes, and every couple minutes forever until you quit to make it go away. If you acquiesce to update it will ask you to update again the next day or maybe at most a few days later. If you ignore the popup and leave it on your screen (this is true!) the dialog box requires 100% of a CPU core just to sit there on screen.
It's completely ridiculous: stable software does not need to be updated twice a week, it needs to be updated twice a decade. If the security of the whole platform is compromised by a bug or weakness in old versions then the solution is to refuse to connect to those old clients altogether which will then force the users to upgrade. And if you aren't willing to refuse the connection then how bad is the weakness really?
Enough carping, the solution is to shitcan updates2.signal.org maybe using this entry in your hosts file.
I'm now looking for the same way to do the same thing for Slack which has the exact same anti-user lack-of-feature.