My understanding is the way PAYG using contactless bank payment card (credit or debit) works is that all touches are reported to the backend processing system, and at the end of day, these are analysed to work out journeys have been made, hence what fares are due (allowing for capping, etc.), and then the bill is sent to the bank/payment card system, which hopefully pays it - if not, the card is (can be) blacklisted to prevent future use until the matter is resolved. Something to be noted is that until the backend processing has happened, the railway/TfL has no knowledge of what journeys are being, or have been, made.
While - in theory - there is nothing to stop bank payment cards being associated with railcards, and a discounted fare being calculated, what I believe cannot be done is for anything recording this to be written to the bank card, and - more generally - there are all sorts of issues (legal, privacy, etc.) to be resolved in the three way relationship between railway, card issuer and card holder/passenger (as an individual): to what extent these have been approached and explored I have no idea.
The 'new generation', account based, Oyster card referred to under 'Project Proteus' will, while working in many ways the same as bank cards in calculating a daily bill, have the advantage that it a railway/TfL product, subject to their conditions (including the right to write what they will to the card, and to say how it can be used), without any need to refer a third party, including whether and how it can be linked to railcards, etc.