If your code is in any way public, it will be trained on. That ship has already sailed.

"all our social security and credit card numbers have been leaked multiple times, so why try to keep it secret anymore?"

More like

"we keep publishing all our credit card numbers for anyone to see, why do people keep taking and using them? :("

If you don't want your code to be trained on, perhaps don't make it public in the first place. Even GPL is fine with this, you aren't required to put it on the internet, you just need to send the source code to the requester, e.g. via physical media as they did in Stallman's time.

If that is your problem with GitHub, then I agree, you should avoid GitHub, though someone can still mirror your repository there. I assume most new FLOSS projects that want to have users don't consider it a dealbreaker.

If your problem is with your code appearing in training data, then you cannot release your code anywhere.

That link you provided only points out GitHub has integrated "create pull request with Copilot" that you can't opt out of. Since anyone can create a pull request with any agent, and probably is, that's a pretty dated complaint.

Frankly not very compelling reasons to ditch the most popular forge if you value other people using/contributing to your project at all.