With Drupal 8 reaching its end-of-life on Nov 2, 2021, you must upgrade to Drupal 9 before November to keep your site secure. Let's have a look why migrating now is best option for your web platform.
Drupal 9 is not being built on a new core, but rather in Drupal 8 itself. This means that new functionality will be added as backward-compatible code and experimental features.