It's simple: updown.io requests cannot appear in Google Analytics (and most other javascript-based analytics like Plausible). For the simple reason that updown does not execute or even load javascript.
updown only performs the HTTP(s) query you configure, but it does not load the page in a virtual browser so it won't query any images, css, javascript, fonts, etc.. (which are sometimes used by other javascript-less analytics tools).
The only kind of analytics where updown requests could be visible, are the ones installed server-side like APM (Application Performance Monitoring) tool, or using server access logs. Because to them the updown HTTP request will be visible. If you do see updown requests here, and you want them to be filtered or flagged but it's not already the case (most of these tool have good User-Agent filtering), then you may configure you tool to flag or filter updown requests based on User-Agent (ideally) or IPs.