There's now an explanation for this change:
https://www.moneysavingexpert.com/news/ ... han-expec/
After 29 March 2019, if the UK leaves the European Union without a deal, Brits will be considered 'third country nationals' under rules used in the Schengen area - an area made up of 26 European countries that have officially abolished passport and all other types of border control at their borders.
Being 'third country nationals' means we will need to comply with different rules to enter and travel around the Schengen area.
According to the Schengen Border Code, if you are from a 'third country', you need to have a passport issued in the last 10 years in order to travel to a Schengen country - and under the old rules, where time left on your passport was added to your new one, not everyone with a valid UK passport would have had their passport issued within the past ten years.
So - Brexit then.
There is a window between 29th June and 10th September where those nine months will have been added to a new passport, which will fall foul of the rule after 29th March 2029.
But, rather than doing away with the time extension, why not just issue passports between now and 29th March 2019 to expire on 29th March 2029, with an ever-shortening extension so no-one will fall foul of the Schengen Border Code, yet the extension is not lost suddenly and without warning?