This syntax is only supported in Vim 8.2 or higher, so this means older
Vims won't work properly, but I don't really have much of a reason to
use older Vims than that anyway.
Note that because plugins.vim now requires Vim 8.2+, it doesn't need to
bother providing special support for Vims that don't have the packages
feature, since packages were added before Vim9 script anyway.
It's a better place to store most of the stuff Vim wants to store lying
around on the filesystem than XDG_CACHE_HOME, it turns out? I'm still
keeping installed packages primarily in XDG_CACHE_HOME, since if that
directory is blown away then the same packages just get reinstalled
automatically and nothing particularly important is lost, but the other
stuff doesn't really belong in there.
Undo files, viminfo, and so on are definitely a good example of "state
data that should be persisted between (application) restarts, but that
is not important or portable enough to the user that it should be stored
in XDG_DATA_HOME", so XDG_STATE_HOME is a fine place for them.
vim-apathy automatically sets Vim's 'path' option for various file
types, which helps a bunch of Vim commands usefully discover included
files (gf, :find, [i, and more).
vim-dadbod provides a :DB command, which can send queries through to a
wide variety of database management systems. This includes Redis, which
is awesome.
vim-eunuch provides Vim commands wrapping various UNIX functionality.
The most exciting are :SudoWrite and :SudoEdit, allowing a privileged
file to be edited and saved without having to 'sudo vim' up front.
gruvbox is updated more often than Inkpot and works better in modern Vim - however, the 'termguicolors' option doesn't quite work correctly in Vim and won't give you a transparent terminal background, so we still use Inkpot in that case.
In Neovim, 'termguicolors' works perfectly - and in GVim it's moot since you're not using a terminal anyway. ;)