Is there any addon that has already used SPDX-License-Identifier in their LICENSE.txt?

There's currently none yet however it's pretty straightforward
Here's a list of identifiers
https://spdx.org/licenses/
For addons they should use something like
<license>GPL-2.0-or-later</license>
in the addon.xml and in the python file itself

Copyright (C) 2005-2018 Team Kodi
This file is part of Kodi - https://kodi.tv

SPDX-License-Identifier: GPL-2.0-or-later
See LICENSES/README.md for more information.

We should also encourage that this is added to every python file.
It should only be a warning/suggestion.

@MartijnKaijser by GPL-2.0-or-later you mean GNU GENERAL PUBLIC LICENSE Version 2, June 199 right?
Also since License will stay same forever,Will hardcoding the License text be a problem?

IMO license tag in addon.xml should be made mandatory before warning about python files which miss a SPDX-License-Identifier-

I'm with rechi here, and we need to document this in the wiki first

If add-ons are needed to test against, plugin.video.youtube, plugin.video.twitch and script.module.python.twitch are all using SPDX license identifiers.