Custom field type: make only one specific property translateable
content_translation has a feature that synchronizes certain property between translations, see how the \Drupal\image\Plugin\Field\FieldType\ImageItem field type uses that:

 *   column_groups = {
 *     "file" = {
 *       "label" = @Translation("File"),
 *       "columns" = {
 *         "target_id", "width", "height"
 *       },
 *       "require_all_groups_for_translation" = TRUE
 *     },
 *     "alt" = {
 *       "label" = @Translation("Alt"),
 *       "translatable" = TRUE
 *     },
 *     "title" = {
 *       "label" = @Translation("Title"),
 *       "translatable" = TRUE
 *     },
 *   },


By not setting translatable = TRUE, the UI by default suggests to keep that value across translations, although it can be configured differently.