On the other hand, I closed an account with a CU some time before they merged with another CU. I now get statements from the new CU saying I have no money in my account. This was after plenty of time of not getting such statements from the old CU. If the new CU is sending out empty balance notices to every single previous customer of the old CU, they’re wasting a lot of paper.
My experience with First of America -> National City -> PNC was absolutely fine. I ordered a ton of checks while they were National City when I was paying multiple bills per month that way, but I now have nothing that regularly needs to be paid by check, so it only happens for one-off things or very rare recurrent things like passport or driver’s license renewal. It might be decades before they’re gone. As far as I know, the checks with the National City information will work fine forever. (Although make sure to use the old routing number with the old account number - there almost certainly will be a new account number associated with the new institution’s routing number.)