The option that allowed users to go back to the UI from before the redesign has been deleted, and the new Gmail design is now the default for all users.
The new design of Gmail is now the default for all users, so you should start getting used to it.
Google still let people use the old interface for its email service even after it started offering a new one.
You now have two interface choices, and if you don't like the new appearance you created, you may stick with the previous one.
The integrated view with Gmail, Chat, Spaces, and Meet on the left side of the window will also become familiar to those who have Chat enabled.