is it true that working dogs cannot be fed raw in case they damage the game they retrieve?  it sounds possible but how did the dogs manage before dry food?

No, it's not true, a soft mouth has nothing to do with being raw fed, dogs still have to chew kibble.  A soft mouth, many believe, is vastly down to genetics, but a hard mouth can be exacerbated by bad handling, and inappropriate toys like squeaky toys etc.  How much of that is true I've no idea, I have one bitch I did working trials training with who has quite a hard mouth, and is too exciteable to do any real gundog work, and the other bitch that I can play tuggy with, yet she will still carry a whole raw egg off down the garden to play with, before eating. 

I know quite a few people who raw feed their dogs and work them, none of them have problems with their dogs snacking on the game on the way back ;)

slightly different I know but my gsd is raw fed and recently was in the company of another breed not raw fed who caught a rabbit ( not something we liked or expected) and returned it to the owner dropping it at their feet. I quite expected my dog to take advantage of a free dinner but to my surprise just sniffed the carcass and walked off  I was relieved I might add !

I therefore presume that dinner  and working meat are two separate things on a straw poll of one !

My dogs are raw fed and the Golden Retriever has caught a squirrel a few times but made no attempt to eat his catch.

No it is completely untrue and utter balderdash!