Add column to custom field schema
In fact, like it's a custom field type, your change in the schema can be detected and applied if you launch the drush command drush entity-updates (or drush entup).

All tables which implement this field (like node__field_my_field_type or paragraph__field_my_field_type) will be updated. So you don't need to use an hook_update to update the schema of all these tables.

The big problem with this command is many errors are silent and sometimes, the command don't return errors but the schema hasn't been updated and still proposed by the command to be updated.