It's a great pity that successive DPAs haven't included an offence of misusing data protection as a convenient excuse because this is just another attempt at it.
GDPR allows data to be kept as long as it's required for its original use. Assuming that the records' original purpose was to prove legality of residence then they remain a required document for the life of the individual to whom they apply. If they were needed to prove that legality for a dependent then they're required for the life of that person too. There's anecdotal evidence that they were still being referred to which should have clarified the matter.
One aspect that's not been mentioned is whether these were statutory records. If they were not only might there have been a statutory requirement to keep them but GDPR wouldn't apply, at least not until any statutory requirement had lapsed. Perhaps this aspect should be looked into further as whoever took the decision to destroy them might have committed an offence.