It seems like this is an unusual use case that OP had an MIT-licensed open source project whose source only exists in a private repo.
- you fork a public repo
- it's visibility is changed to private but you have access through the org
- they delete it
- you lose your fork, which was only ever of the public repo
"GitHub will detach public forks of the public repository and put them into a new network. Public forks are not made private."
https://docs.github.com/en/repositories/creating-and-managin...
But better be safe than sorry...
It seems like this is an unusual use case that OP had an MIT-licensed open source project whose source only exists in a private repo.