Today alone I've had to do this process twice , I reckon I've sent copies of my credit/debit card to over a few hundred different bookmakers. I am/have been a member a customer of a few hundred different bookmakers/casinos in the last few years and this is requested alot probably about 25% of the time , I've never had any implications from sending it in, nearly all the emails they send asking for it state you can block the middle 8 numbers.
Some stranger requests I've had from bookmakers/casinos
Please send a photo of yourself holding your passport/driving licence
Please send a photo of yourself holding todays newspaper in one hand and your passport /driving licence in the other , please make sure we can see the date on the newspaper and also the date of birth and photo on the passport and your face holding them up.
I've argued over the years that bookmakers should do there ID checks before they let you deposit but they would never do it , when you go to withdraw its when you get problems , most books are regulated in Malta or Cuaraco or Isle of Man , mainly Malta , you have to contact the LGA(lottieries and gaming assocition in Malta) and they take forever to get back to you, I had 20k stolen from me from a well known bookmaker and I am 2 years waiting for the LGA to look into it, send the documents they requested is my advice its the easiest way.