In the self hosted instances, which is what I was talking about, you do have a control over the backend. At last more than you do with the managed version. It pretty much boils down to this: Can something leak sensitive information to a third party. If so, then it's a no go.
If you have a contract with that third party, and you deem that third party to be a safe harbour for your data (yes, that includes gitlab.org, AWS, etc), then that's a different case.
If Gitlab was to have instead said:
1. We are going to enable telemetry on all public repositories on Gitlab
2. On self-hosted instances we will provide you with the ability to embed your own analytics, from a company of your choosing
Then the screeching (and I fully agree it is screeching) would have been less. Unfortunately, with self-hosted instances, you simply cannot allow Gitlab to leak information like that to a company you don't have a direct relationship with. I'm not sure how else to phrase this concept or explain it, and it doesn't really matter if you use safeframes or not.