The grid layout type for UI list is planned for removal in 5.0, see
blender/blender#110461.
In previous UI meetings, we talked about deprecating the Grid mode of
the UI list, which is not actually accessible in UI and was never used.
Nowadays, there is a new grid view that can be exposed in the API in
the future.
Initially, I wanted to remove references to layout_type in UI templates
in the text editor, because a lot of add-on developers on the
extensions platform base their lists on that code, and a lot of them
are therefore including soon to be deprecated code in their add-ons,
which I want to avoid in the future. But I thought we might as well
remove it from our python scripts as well, since it's just basically
redundant code that doesn't do anything. And also because many add-on
developers use bundled python scripts for references as well.
Pull Request: https://projects.blender.org/blender/blender/pulls/138395
efa8d942b8