Recently, I needed to add some dynamic content to a Views footer. Specifically, I needed to change a link in the Views footer based on the current path, which isn’t an option from the Views UI. I found some good documentation showing how this can be done in older versions of Drupal (https://www.drupal.org/node/749452), but nothing for Drupal 8. I figured the approach must be similar in Drupal 8 so I started searching and reverse engineering.
Posts in Drupal 8
In case you've had trouble tracking it down (I did), the approach to caching custom blocks is different in Drupal 8 than in Drupal 7. In Drupal 8 all renderable arrays are cacheable – including, of course, those returned by custom blocks.