The following books were published during Siegfried Sassoon’s lifetime and Keynes collated 34 of them, I have found over 100 so far. There are a great many that Keynes missed and I am coming across them regularly, particularly poetry anthologies. The compilers of these anthologies must have asked for permission to reprint the poems and it is quite possible that there were so many that Sassoon was unaware of most of them.
It is equally likely that the compilers contacted Sassoon’s publishers to ask for permission to reprint the poems and any payment charged was split with some of it being paid into Sassoon’s bank. The chances are that Sassoon knew little of these transactions and therefore did not know how often he had been anthologised!