How to set a transparent background for Vim with "true color" turned on?

Question

I have successfully used true colors for Vim using:

set t_8f=\[[38;2;%lu;%lu;%lum
set t_8b=\[[48;2;%lu;%lu;%lum
set termguicolors

(check :h xterm-true-colorfor more information)

Then I turned on the true color color scheme to try a new setting, for example:

set background=dark
colors deep-space

( https://github.com/tyrannicaltoucan/vim-deep-space )

Earlier, when using 256 colors, I managed to get a transparent background:

hi! Normal ctermbg=NONE guibg=NONE
hi! NonText ctermbg=NONE guibg=NONE

Now it no longer works, and I do not know an alternative. Is it possible to get a transparent background using Vim under termguicolors?

UPDATE

I discovered the problem in the Vim issue tracker: https://github.com/vim/vim/issues/981