Does a stop payment on any type of transaction (check, recurring ACH, recurring debit card trans., one-time ACH) expire after 6 months?

Thanks in advance.

Typically yes - but stop payments are a State law issue.

ACH allows for both a one-time and permanent stop...a permanent stop should never expire, but the customer can rescind it

What about a preauthorized transfer via debit card to an account...wouldn't a stop payment also never expire and so that is why FIs cancel the debit card?

Under Reg E, the consumer has a right to stop payment a preauthorized EFT with no expiration date for the stop. That includes recurring ACH or debit card transactions.