It's correct to say visitors shouldn't deal with £50 notes if they can possibly help it, because they will generally struggle to pay for anything with them though..
Anything over a $20 bill has been hard to use in many/most stores for a very long time in the US (which seemingly at one point decided to freeze currency denominations in amber--see also penny--which I assume is permanent at this point as less and less cash gets used). But $100 bills at least were in pretty common use overseas. I still tend to carry some as backup cash.
£50 notes were popular for criminal transactions, and also they were widely forged; I don't know whether that's still the case, but I'm very reluctant to accept a £50 note. If a shopkeeper checks it and decides it's a forgery, they have to take it off me for nothing, and I'm down fifty nicker.
It's also incorrect to say visitors won't deal with £50 notes. They're rarely in ATMs but they are common when exchanging cash abroad.