Last I checked, Twitter supports U2F, but only allows enrolling one key.

edit: I guess the thread is referring to multiple fallbacks that aren't U2F, but even still, if you're relying solely on U2F it's good practice to have more than one key lest you lose it and get locked out.

I haven't kept a running list, but just checking a few services I use at work, and Terraform Enterprise doesn't appear to support more than one 2FA method being enabled at a time.