Nothing new these notes are being replaced by new polymer notes which are kinda becoming standard in many countries.
This occurs regularly in many places, this is also to avoid counterfeit notes where the central bank replaces old notes with new notes with additional security features after a certain amount of time. Usually it happens in the back end but sometimes they put an expiry date too.
But in this case you will not be able to use these £20 and £50 for shopping etc, but you can always exchange old notes at the Bank of England with new notes.
The Bank of England will always exchange any withdrawn notes, including paper notes we have withdrawn in the past.
https://www.bankofengland.co.uk/banknotes/exchanging-old-banknoteshttps://www.bankofengland.co.uk/banknotes/paper-20-paper-50-withdrawal