Do they now require a membership to buy from the snack bar in Oxnard?

Correct! It's part of social distancing and limiting the number of people coming to the store.

Yes

Yes, a membership has been needed at the food windows since mid-March 2020

I've never heard of that but it is possible. If you have a Costco Cash card you can make a purchase in store and at the snack bar without a membership

Yes they do.