Policy has always been there, so my guess is historically drivers were turning a blind eye, and that's what's been stopped.

TFL CoC dated November 2022 (my bold and italics):

9.1 For safety reasons, and for the comfort of other passengers, we have to control what you
can bring with you onto our services and property, although we do not charge you for the things
we allow.
Our services can be busy, so you must ensure that what you bring with you does not get in the
way of others.
You must ensure that what you bring with you, such as your shopping or luggage, does not block
a seat, the aisles and doors, stairs, lifts or passages.

9.2 Staff can refuse permission for you to take any item onto our services.