Read the Shimano tech docs, it tells you what cassette teeth patterns you can sue with what, however 10-speed shiamno MTB and 10-speed road use different cable pull ratios and are not interchangeable, to run 10-speed road mech you have to use 10-speed road flat bar shifters.
I don't think you've gone through those tech docs or the first post. The RDs mentioned in this thread are marked as 28t max but yet they are handling 32...
BTW XTR 970 shifters are compatible with DA7900 RD
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