I wanted to create a WP plugin. So I requested and my request was approved by WordPress.org But I don't want to develop that plugin anymore. How can I delete the repository forever or request its deletion.
Please mail plugins - wordpress.org and ask them to remove your plugin. Provide all infos and explain that you won't start to develop the plugin so the name should no longer be blocked.
I would use the same email as while registering the plugin (maybe even replying to it) and I think they will remove it after some days.
From what I've heard, you can't really remove it from the repository. Besides that, you really shouldn't. Someone somewhere might still be using the code and might need the repository for reference. Or it might be a good educational tool later on down the road.
But if you want to stop supporting it (i.e. abandon the plug-in), there is something you can do ...
Upload a new readme file to the repository explaining that you're no longer developing or supporting the plug-in. Make the message large and the first thing people will see in the readme. Also make sure you put a reference in the description of the plug-in's core PHP file explaining that there will be no future versions.
If, in the future, another developer wants to take over you can always assign them as the owner of the repository. Otherwise, it will just sit there in the archives with its "This plug-in is no longer supported or being actively developed!" message to warn people away.