I’m sorry to hear about your rabbit’s passing. Binky free bun.
This may be sore hocks, though not in the “traditional” location: https://wabbitwiki.com/wiki/Sore_hocks
Unlike other animals, rabbit feet have no padding or extra cushioning aside from their fur, therefore improper flooring, improper nail length, being overweight, being sedentary, etc. are all factors that can contribute to the development of sore hocks. Some rabbit breeds, like Rexes, are more prone to sore hocks since their fur is short, providing less cushioning.
If it is sore hocks, this does not look too bad at this point, but I’d consider reviewing if you can make improvements in any factors listed in the Wabbitwiki link and check the sore each day to see how it progresses. If it increases in redness, size, or bursts, veterinary assessment would be wise.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.