If they've gotten rid of AA transfers on personal US cards, chances are they've downgraded the method / frequency of transferring to AA.
I know to some more obscure partners in the past they only transferred once or twice a month, and using physical media (tapes, discs, whatever) that had to be mailed and that sometimes misread, etc, but the data was duplicated on several successive transfers, so if one was corrupted the next one would pick it up. (No, transfers are not done individually and electronically, as you might think. They're done in bulk, and are often still done physically.)
Anyway, if AA has been downgraded to an "obscure" partner, it could be they've reduced the frequency of transfers and/or changed the medium.
Keep in mind that Citi has an extroardinarily close relationship to AA (Citi is AA's bank, not just for credit cards and such, but for actual funding of the airline!). Now that Diners is not longer Citi, it wouldn't matter who had taken it over, I would excect a not-as-close relationship with Citi with any possible new owner unrelated to Citi.