bug: window separators are not highlighted correctly on Neovim master

Neovim version (nvim -v)

NVIM v0.10.0-dev-1773+gca7f8786a

Operating system/version

Fedora Linux 39

Describe the bug

VertSplit is a deprecated highlight group that was superseded by WinSeparator in https://github.com/neovim/neovim/pull/17266. But WinSeparator was linked to VertSplit which meant that setting VertSplit still worked. However, the default Neovim colorscheme was updated in https://github.com/neovim/neovim/pull/26334, which removed the highlight link. Which means that setting VertSplit no longer works.

Steps To Reproduce

Just use the theme in Neovim master and open a split window.

Expected Behavior

Window separators are highlighted correctly.

Repro

-- DO NOT change the paths and don't remove the colorscheme
local root = vim.fn.fnamemodify("./.repro", ":p")

-- set stdpaths to use .repro
for _, name in ipairs({ "config", "data", "state", "cache" }) do
  vim.env[("XDG_%s_HOME"):format(name:upper())] = root .. "/" .. name
end

-- bootstrap lazy
local lazypath = root .. "/plugins/lazy.nvim"
if not vim.loop.fs_stat(lazypath) then
  vim.fn.system({ "git", "clone", "--filter=blob:none", "https://github.com/folke/lazy.nvim.git", lazypath })
end
vim.opt.runtimepath:prepend(lazypath)

-- install plugins
local plugins = {
  {
    "EdenEast/nightfox.nvim",
    config = function()
      require("nightfox").setup({
        -- setup here ...
      })
    end,
  },
  -- add any other plugins here
}
require("lazy").setup(plugins, {
  root = root .. "/plugins",
})

vim.cmd.colorscheme("nightfox")
vim.cmd.vsplit()

EdenEast / nightfox.nvim