IME a 9spd chain will work on a 10spd cassette IF the cassette has no single tooth steps, ie 11->12 or 12->13. If the
cassette has 2 tooth jumps ie 11->13 then the 9 spd chain will work. So very wide range cassettes with no single
tooth changes cog to cog will work fine with 9 spd chains. The problem with single tooth changes is that the next cog
will try to pick up the chain resulting intermittent up then down shifts as the chain bounces up and down on the
cassette. Two or larger tooth differences mean the chain is far enough away vertically from the next larger cog that this does
not happen. This experience is with Shimano/SRAM cassettes.