The Mail app offers two ways to view your mail - either by Categories or by ListView. If you choose Categories your Inbox mail is automatically classified into categories named Primary, Transactions, Updates, Promotions and All Mail. You must select the category you want to view near the top of the Inbox screen. By default, it will be Primary. VoiceOver users will be told which category is selected but won't be told that the category labels are buttons that can be double tapped.
I'm sure that many people will find Categories helpful but it can be confusing and cause you to overlook or unintentionally delete messages if you don't understand how Categories works. I think that VoiceOver and low vision users will need to take a careful look at Categories to ensure they understand it and want to use it. I recommend that you turn it off initially and make your decision only after you take time to experiment with it and discover if it's for you
If you want to choose either Categories or List View, use the more button, three dots in a circle, at the top right of the Mail Inbox screen. A menu will pop up. To choose the way Mail used to look, select List View from that menu. VoiceOver users can find the more button by swiping right a few times from the top of the Inbox screen or by exploring near the top right corner of the screen. After double tapping the more button VoiceOver will be focused on Categories. To select Categories double tap now or to select List View swipe right and double tap.
Here's a little more information about Categories which may help you to make your decision.
Mail in the Primary and All Mail categories behaves much the same as it used to do but the other categories are a little different. By default, the other categories behave like the conversation view in the Messages app. Messages from the same sender are grouped together and only the most recent message is immediately visible. If you choose to delete in this view you will be asked if you want to delete all messages in the group. If you open the group, to view all messages from that sender then you will be able to view and delete individual messages. I'm concerned about the group deletion behaviour because I think it's too easy to delete important messages unintentionally this way. For example, a message with my tickets for an Eventbrite event was grouped together with later marketing messages from Eventbrite and it was one of those later marketing mesages which showed in the list. I could easily have deleted the ticket unintentionally. If you like Categories but have concerns about grouping, it's possible to turn off grouping on a category by category basis.
If you decide to use Categories and want to turn off the grouping behaviour this must be done for each of the three non-Primary or All Mail categories by using the more button at top right of the category's screen and turning off Group by Sender in the pop-up menu that appears.