Visa were doing this before Brexit…
I don't know about credit, but my UK Visa Debit, when going to German cash machines was always a very low fee. Now, I will add a caveat that I haven't used it over here (at a cash machine) for a few years and I think I had to use it once last year but all good, no extra charges other than conversion rates.
However, banking in general. I have to send money back to my UK account fairly regularly. Early January I tried and it failed, the bank advised me it was directly because of Brexit. I am able to use intermediary apps to transfer from Germany to UK now, luckily I am technically savyy to have apps and get around the larger fees and blockages but not all are and it can affect peoples lives.
Brexit is a pain in the arse and there is absolutely no logical reason for it to happen, especially when considering the people that are affected by these changes.