<C-W> is commonly used in shells to delete a word. If there is a Terminal mode mapping that starts with <C-W>, one will have wait 'timeoutlen' after typing <C-W> for the word to be actually deleted, which is a degrade in user experience.

Not a core contributor so I dont know if my opinion counts for anything.

I request to please not map things to Alt as default. Alt+[key] can and will conflict with many OS/desktop-environment/Window-manager defaults for many of us. Having to unmap new nvim defaults for just this reason is a bit much.
Besides Alt is just one of those avaiilable global keys, has nothing to do with window pattern such as C-w prepend.

Also, while its true that most people map the terminal mode keys to something of their choosing, why not just let it be example in docs and/or suggested minimal_init.lua, why enforce it?

Alt+[key] can and will conflict with many OS/desktop-environment/Window-manager defaults for many of us

Why aren't you using super ("Windows" key) for that? It is a good choice for that.

We need a common chord prefix for window-related commands. There are many more similar commands that could use this common chord, such as ALT-n for :enew, ALT-t for a new tab, etc.

Having to unmap new nvim defaults for just this reason is a bit much.

To unblock this, I'm in favor of some sort of option like g:default_mappings, or even a command like g<BS> (for the nvim --clean case). It's that important. We can't be blocked on this decision for eternity.

Alt + key is a very common mapping for tiling window managers. It’s also used for many keymaps in Zellij. And I personally use it as a prefix for keymaps in my terminal emulator.

I think that's only a problem if you run Zellij inside Nvim terminal? If you run Nvim inside Zellij, key sequences have to go through Zellij first, don't they?

To unblock this, I'm in favor of some sort of option like g:default_mappings, or even a command like g (for the nvim --clean case). It's that important. We can't be blocked on this decision for eternity.

What about <Plug>(nvim-window-prefix) (or just <Plug>(nvim-window))? For example, if users only want all those window-related mappings for normal and insert mode, but not terminal mode, they just need to run ONE line of Lua

vim.keymap.set({ 'n', 'i' }, '<M-w>', '<Plug>(nvim-window-prefix)')

instead of setting g:nvim_window_map and then run N lines of Lua/Vimscript to unmap N mappings for Terminal mode.

Saying + HJKL is "not ergonomic" is like saying something is overrated. That key combo has been the standard since the dinosaurs use Vim. It’s part of the Vim tradition: consistent, easy to remember, and extremely well-documented.

Alt + key is a very common mapping for tiling window managers. It’s also used for many keymaps in Zellij. And I personally use it as a prefix for keymaps in my terminal emulator. Adding those keymaps by default will cause a lot of conflicts with user environment keymaps for no benefit

Your window manager, terminal emulator, and Zellij will all "see" the key presses before Neovim ever does, so you wouldn't need to unmap anything. The Alt prefixed key sequence would only reach Neovim if it wasn't first consumed by another application.

Alt + key is a very common mapping for tiling window managers. It’s also used for many keymaps in Zellij. And I personally use it as a prefix for keymaps in my terminal emulator. Adding those keymaps by default will cause a lot of conflicts with user environment keymaps for no benefit

Your window manager, terminal emulator, and Zellij will all "see" the key presses before Neovim ever does, so you wouldn't need to unmap anything. The Alt prefixed key sequence would only reach Neovim if it wasn't first consumed by another application.

Didn't think about this, so it isn’t a big deal then.
But anyway, what problem would adding those keymaps as default solve? Four fewer lines of code for the user?

But anyway, what problem would adding those keymaps as default solve? Four fewer lines of code for the user?

It opens up an entire set of window/layout-related keymappings, like I already mentioned. There are 10-15 mappings that I will propose, probably more.

Your window manager, terminal emulator, and Zellij will all "see" the key presses before Neovim ever does

So that objection is unblocked.

And as @brianhuster mentioned, defining these with a <Plug>(nvim-win) prefix is a low-cost way to let users opt-out, or to allow us to revert by changing 1 line later.