Since the update 18.0, 2021, the old layout options for 3D has been removed. The old (17.5) used to look like the picture.

Now you have options for:
1 views
2 views
4 views

You used to have options for:
1 views
2 views (horizontal/vertical)
4 views (top/bottom/left/right)

4 views - TOP for example (as in the picture) would then allow for a big "main" window and three smaller windows.

Why is this necessary to still have?

When I (and I guess a lot of other people) work with 3D in AE, we merely use for example the TOP and RIGHT view for reference to align things properly and make it look good in the main camera view. Therefore, all windows don't have to be the same size. Also, when using the option for "4 views - LEFT or RIGHT", it's much more user friendly to work with vertical formats such as 9:21, 9:16 and 4:5 for social media.

And while we're on the topic. An option for "3 views" would also be pretty cool and great to have.

Thanks!