They anonymise all PAYG data after 8 weeks. They do not discard records of travelcards purchased and the Oyster cards they are stored on.
You clearly are correct and nobody doubts that this is the case
However this is not what TFL state is their policy regarding retaining journeys after 8 weeks on travelcards which is where I see the issue
( note they say individual journeys made using your oyster card. No qualification of PAYG or otherwise)
https://tfl.gov.uk/corporate/privacy-and-cookies/oyster-card
Choosing not to provide personal information
You can use your Oyster card to pay as you go at any time without providing your personal information to TfL. You can also add weekly and monthly season tickets to your Oyster card without needing to register your card or create an online account.
If you choose not to provide personal information to us, you will not be able to open an online account or get a refund if your card is lost or stolen. It may also delay or prevent us from offering other services to you such as the resolution of complaints or concerns.
TfL will retain personal information in line with its data retention policies. This means that we will not hold information for longer than is necessary for the purposes we obtained it for.
We retain data about the individual journeys made using your Oyster card for eight weeks after the card is used. After eight weeks, the journey data in the ticketing system is disassociated from your card (ie pseudonymised). This eight-week period is considered reasonable to enable customers to verify or make enquiries concerning their journeys (for example, for refund purposes).
Some journey information is also stored on the Oyster card itself; this comprises the last eight journeys and related charges, up to three season ticket products, (generally the most recent three tickets, including future dated), and the last two incomplete journeys, if any. If you don't use your Oyster card very often, the data stored on the card may be older than eight weeks.