Do all Banks and Credit Unions in the U.S. accept Bitcoin for deposits or withdraws?
Certainly all banks and credit unions do not accept bitcoin, and I'm not aware of any that do.
Banks and credit unions can be thought of as nodes in their own network that is not actually so dissimilar to Bitcoin. The function of this network is to create, store, and transfer money. Most fiat currency is not created by the central bank, but is lent into existence by the process of fractional reserve banking. In other words, a bank is inherently part of the currency it operates with. It plays a role in the money creation process.
With this view, your question is similar to asking "can the Litecoin network transfer bitcoin?", or "can an Ethereum miner create a Bitcoin block?". These are separate networks, operating with different sets of rules. The same is true of Bitcoin and the traditional financial network.
That said, many traditional financial institutions operate small-scale foreign currency exchanges for the convenience of their customers. You can often go up to a bank teller in the US and exchange a small amount of euros (leftover from a trip abroad) for dollars, usually only if you are a customer or memeber. I do not know of any traditional bank or credit union that has added bitcoin to the list of currencies it exchanges, but I doubt there are many...if any at all.