I've now received official notification from TfL of the new rule:

From Monday 15 June, we are making temporary changes to the 60+ London
Oyster card and the Older Person's Freedom Pass during morning peak
hours. These changes are being introduced following TfL's funding
agreement with Government which required temporary restrictions for
customers with these concession cards.

Passengers with a 60+ Oyster card, Older Person's Freedom Pass, or
English National Concessionary Scheme pass will not be able to use
these cards before 09:00 on weekday mornings. This is to enable social
distancing on the public transport network and help control the spread
of coronavirus. Your card will continue to be valid at all other times
on weekdays and all day on weekends and Bank Holidays. Disabled
Freedom Pass holders are not affected by these changes and can
continue to use their card at all times.

Where possible, we ask you to avoid travelling immediately after 09:00
to help reduce crowding in buses and trains, where the available space
has effectively been reduced to allow for social distancing. If you
have no option other than to use public transport during the morning
peak period, we recommend that you travel using pay as you go with a
contactless card or device or an Oyster card. For more information,
please visit www.tfl.gov.uk/fares.

We thank you again for all your help as we work together to keep
everyone safe.