I've had to claim lots of times from Thameslink and EMR, because they have so many delays and cancellations, obviously. I've never been asked to deface a ticket that I've scanned/photographed but they are nearly all day tickets which are already out of date at the time of the claim. The only claims that I've had rejected are e.g. when I thought I was 15 minutes late but they say only 14.
If I know that my train is late I always ask at the manned exit barrier if I can keep my ticket for a Delay Repay claim and I've never had that refused - I recommend everyone doing that as it simplfiies the claim.
I've also had a successful claim from Thameslink when using my credit card contactlessly, but when the only evidence I had was a credit card statement, because the TfL server has been down since their hacking attemp many weeks ago. This took longer, I guess it had to be approved manually, but I got the money eventually.
I also had a good experience recently when we got delayed on a Piccadilly line train from Heathrow to King's Cross - lots of Picc line problems in recent weeks apparently. My Oyster card record showed that I was over 20 mins late into London for which I expected some small amount of compensation, but they refunded me the whole cost of the journey, which was generous.