Github repo is not properly mirrored
Please fix this for proper Plugged integration in the README and update the github code. For example the last commit on gitlab is 04 Feb, 2020, but on github on Oct 25, 2018.
The alternative is to provide a direct gitlab link:
Plug 'https://gitlab.redox-os.org/redox-os/ion-vim.git' "Syntax highlighting plugin