Migrating multi-value non-associative data

Migrations in Drupal 8 are very stable and highly extensible. Still there are many cases though which would require custom code. I came accross one such scenario recently and will share some knowledge here which I gathered while working on it.

One - most of the examples shared are around process plugin or the plugins available in migrate (part of Drupal core) / migrate_plus modules. Ones that I browsed:

Drupal 8: Updating field configuration in update hooks

Continuing to Reverting features in update hooks, I encountered a new scenario yesterday where I had to make a field translatable. Somehow we missed to make the field translatable during development and now since we are in update mode we need to handle it for existing sites as well.