Is that a best practice in hindsight, or because it was known to some, that this issue exists, or for what other reason do you consider it a best practice? Git history?
When making a private repo public, there's a high chance that there was stuff in the private repo that isn't necessarily ok to make public. It's a lot easier to just create a new public repo containing all the data you want to make public than it is to reliably scrub a private repo of any data that shouldn't be there.
More generally, you probably want to construct a new history for the public repo anyway, so you'll want a brand new repo to ensure none of the scrubbed history is accessible.
I worked in Professional Services at AWS for a little over three years. There was a fairly easy approval process to put our work out on the public AWS Samples (https://github.com/aws-samples) repository once we removed the private confidential part of the implementation.
I always started a new repository without git history. I can’t imagine trying to audit every single commit.
