Upgrade from Drupal 6 or 7 when it is right for you: In the era of modern Drupal, release cycles of major versions have only minor importance
Drupal 7 has a much longer lifespan than the (already pushed back) official date, and Drupal 8 has an essentially infinite lifespan because it can update in-place to Drupal 9 easily and the same will be true of Drupal 10, 11, ∞. There's no reason to rush an upgrade— but there's no reason to wait either.
Keep modules compatible with multiple Drupal Core branches while using new services
How to keep modules compatible with Drupal 8.7 and below while still using the new services.
