As the CU I work for has multiple branches, when a branch needs to close without advance notice (such as a branch employee testing positive for COVID), the closed branch would post a notice on the front doors directing members to the closest open branch, the onsite ATM, the website, our app, and call center, Due to the member having multiple access points the the credit union, we would not have issues with overdrafts.
Also, some credit unions participate in shared-branching, which allows members to make deposits, withdrawals, and sometime loan payments at other credit unions (which is quite handy when you are traveling).