This is the real anti-feature. You should only be able to fork a private repo using an account that is directly managed by the organization that owns the repo. That way when you revoke access to the user they automatically lose access to the fork.
It's super weird that it's common to use the same account for work and non-work stuff on GitHub (myself included).
That gets into other aspects of how people are using GitHub.
Using that model, your work on private repos wouldn't show up as "your" GitHub activity history.
Your work on external projects sanctioned by your employer (and thus using your employer's managed account) wouldn't be associated with you when you leave. For example, if you were at VMWare and contributed to the Spring project - if you left VMWare the "I did core work on Spring - its right there in a public repo" would not be associated with the account that you're saying is you.
Yes, it is weird to be mixing work accounts and personal accounts (and the mess I have had with email when my gmail account was associated with a former employer).
There's tradeoffs no matter which way that you do this... and people appear to prefer the set of "using a single account on GitHub for work and personal" and then having the follow on implications of that being that you may lose access to internal repos when you leave... which you would in either case, just its a bit more surprising when its your "personal" account.
Yes, because GitHub doesn't support organization users (maybe you have to pay?) so the thing is that you use your personal account. That is fine, since the real account is the membership to the organization. The only thing is the stuff of private repo: if there is this unintuitive behaviour it shouldn't be allowed to fork private repositories at all.
Or the fork should not be deleted, but it should be made in a way that it's equivalent to a pull and then push to another repo, that is if you loose access to the original private repo you can still see your code, but you can no longer pull from the upstream private repo. I think this is a problem on how data/permissions is represented at low level in a repo, so if this is the case and cannot be fixed they shouldn't allow private repo fork at all.
This is the real anti-feature. You should only be able to fork a private repo using an account that is directly managed by the organization that owns the repo. That way when you revoke access to the user they automatically lose access to the fork.
It's super weird that it's common to use the same account for work and non-work stuff on GitHub (myself included).