#7062 changed the meaning of "descendantfees" without changing the name.
"descendantfees" with the previous (and expected) meaning is about to be released in 0.12, so this will be a regression in 0.13.
If #7062 is indeed a relevant fix, it should be backported to 0.12, but either way IMO this needs to be renamed to convey what it actually is (if #7062 is backported, this should be as well).
Since most users don't, I believe, use prioritisetransaction, I don't think the name change matters at all, though I would have been happy to change it to be more acceptable during the review of #7062.
Seems way too late in the 0.12 release cycle to bike shed the rpc field, and I don't think we should only make the change in master and needlessly introduce incompatibility between the about-to-be -released 0.12 and future versions.
Aside from that concern I'm indifferent to this change.
I'm indifferent to this too, weak NACK.
The mempool code is still in flux, and this is an internal implementation detail that is exposed, next release there may have been changes again. Bike shedding on naming is just a waste of time.