Apparently Apple's new design for tabs in iPadOS 18 app windows is not going over well. Reading these posts I would agree, this looks like a regression. Yuck.

Nico, the developer of the Gamery app has a post, Let's talk about the new tabs and sidebar in iPadOS 18.

Viticci also posted about it, specifically about the design of the new Books app.

Effectively, by eschewing a sidebar, the app has returned to feeling like a blown-up iPhone version – something I hoped we had left behind when Apple announced they wanted to make iPad apps more desktop-class two years ago.