Old Exemption Transcript for a farmer

[GabbyTheKing]

I have a general query that I'm not 100% sure of. I have a farmer Drystock who lets say 20 odd years ago received a letter from the Inspector of taxes that he no longer had to return any tax returns. It was a pretty normal scenario at the time I believe. But do these letters still stand 20 years later? I've never seen one of these letters and his lost it and I'm just unaware of any legislation that covers him for not submitting a Form 11 or 12. I understand Chargeable Person very well with the 5k assesable(from 1/1/2016) income and 50k gross and I'm sure he wouldn't be a chargeable person in any instance but is he open to question in the future or has somebody experience of these letters standing strong?

Thanks for your help.