Did you contact GitHub regarding this and ask them, before bashing them on social media? What was their response? They'll probably gladly remedy the situation.
> Luckily I found an old local copy of my project [...] I haven’t tried contacting customer support, but as this appears to be official policy I would not expect a change there.
The policy itself needs to be changed, the individual case is resolved.
Or the policy was misinterpreted. The piece of text that got quoted is present in every single EULA ever and does not at all indicate if this is actually intentional by GitHub.
Besides, you can request to disconnect your fork from an upstream project. I expect such a request will also resolve this problem.
