Can I set a horizontal layout for workspaces?

There are two extension to achieve the desired behavior:

• Workspace Grid Extension
• Workspace Matrix

The former seems to be more popular (and still seems to receive updates) while the latter is more actively developed.

The Frippery Bottom Panel exension sets a horizontal layout for workspaces. When it is active, you need to use ctrl+alt+left and ctrl+alt+right to switch workspaces, instead of ctrl+alt+up and ctrl+alt+down.

At the time of writing, however, it doesn't change the layout of the workspaces in the activity view, where the workspaces are still stacked vertically, and where you need to use the old ctrl+alt+up and ctrl+alt+down keys.

Also, it adds a bottom panel which you can't remove without disabling the whole extension. On this bottom panel, there are buttons to quickly access each workspace, but they don't show a miniature of the windows on that workspace, like we used to have in gnome 2.

The extension is only 1128 lines of rather straightforward javascript, so I might derive a quick and dirty "horizontal workspace layout" extension in a few days if I find some time.

It's not possible to do this, it would be better for you to ask this of a GNOME developer via their mailing list, and if you know how to code in javascript you could possibly write an extension for it.