Well, it's not reasonable to me, but we'll just have to respectfully disagree on this.

It probably seems more reasonable to the people who own the repo.

I don't think the people who removed my access had any intention of deleting private repos. It's just somebody cleaning up ex-employee access from GitHub.

As a non employee, how would I access this repo?

The most “expected behavior” here would be to remove the fork relationship between the two repos and leave the copied repo as a plain private repo. I don’t think it is reasonable to just delete a private repo by default.