It is over a year since I wrote how to get the current node in a block plugin and promised to write a follow-up post with an easier method. Well, better late than never, here it is!
The previous method works, so why bother with another method? In short, it’s less code and less code is easier to maintain. Also, instead of doing all the work to get the node and set the correct cache contexts/tags, it is better to let Drupal handle that for us.