by Randy Carey: I highly doubt that you will find any supporting documentation one way or the other. The account statements that you will provide them will have that person's name on them, so if they pay any fee you have associated with producing past statements, I would say they are more than entitled to the copies.
This leads to the next issue. Most financial institutions will close a joint account and open single accounts and will never just remove a joint account holder from a consumer account. That eliminates these types of issues.
by Ken Golliher: A primary account holder removed the joint owner from the account.
That's where your problem started. A joint account with two parties, Randy & Ken, is a three party contract. (The parties are the two individuals and the financial institution.) No two of those parties can change the terms of the contract without the agreement of the third. As suggested, had the joint account been closed rather than just altered you would not have an issue; either party would be entitled to copies of the records over the life of the account. Now, the water is muddy.
The "primary" owner may have some special importance in complying with a credit union's field of membership. That does not imply that the first listed individual has some sort of superior rights in the contract unless the contract actually says that.
Once a financial institution starts making up the rules, it is stuck with continuing to make them up. If Ken took Randy off the account and Randy wants copies of prior statements when he was on the account "No" is a made up answer, but so is "Yes." There's more logical support for the latter, but Ken may want to whine about it.