It depends on the outside width of the chain. I used to run Sram PC-89r 9s chains on my Chorus 10s system and never had an issue other than that I finally ran out of those chains.
The most likely issue of running a 9s chain on a 10s group is clearing the inside of the outer sprocket when using the inner with the outer few rear sprockets. Even a 10s chain often touches the outer in the 39/11 combination, but many riders report issues with the 2md or even the third when the chain is a a bit wide.
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