Surely the use of user-deleted content as training data carries the same liabilities as reinstating it on the live site? I've checked my old content and it hasn't been reinstated. I'd assume such a dataset would inherently contain personal data protected by the right to erasure under GDPR, otherwise they'd use it for both purposes. If that is correct, regardless of how they filtered it, the data would be risky to use.
Perhaps the cumulative action of disenfranchised users could serve toward the result of both the devaluation of a dataset based on a future checkpoint, or reduction in average post quality leading to decreased popularity over time (if we assume content that is user-deleted en masse was useful, which I think is fair).