When they merged, US had a CC deal with BOA, and America West had Barclays. Barclays is the only one you can still apply for, but because of terms in the contract, US had to agree to keep existing US BOA card members actively accruing miles on US. I know, because I still have the BOA card.

So, some of it is going to depend upon the terms of the contracts with each of the airlines and financial companies.