Potential developer testing code left in gtag.js

It is supposed to be in there
https://www.drupal.org/project/google_analytics/issues/3328262 →

This is by design. That developer ID is what google uses to identify usages from Drupal. You can see more here: https://developers.google.com/tag-platform/devguides/gtag-integration

I know of at least 3 issues that were opened with confusion on this point. It may be a good idea to add some prominent documentation explaining this to the module page. $.02

Or a comment in the code?

There was a comment once, but it was removed for some reason
It would be good to add that back I think