How do I create a custom plugin?
There are no ctools plugins anymore in Drupal 8. There's just the Drupal 8 plugin API.

Page Manager, Panels and so on use standard block plugins now in 8.x, so you basically have to port this to a block an then it should work fine.

Also, the "code" you posted only defines that you have such a plugin in your module, nothing else. If you need more specific support, you will need to post your actual code, and ask specific questions (which are probably better as a follow-up question).