Group/list/sort custom post type posts by date in tabs from acf datepicker field

It’s probably best to loop through your posts two times. Once to group them by year and once to print the posts.

Something like this: (not tested)

$formatted_posts = array();
while ( $subsQuery->have_posts() ) : $subsQuery->the_post();
  $year = get_field('year');
  // add them to formatted_posts array and group by year
  $formatted_posts[$year][] = $post;
endwhile; 

foreach($formatted_posts as $year => $posts) {
  // print $year
  foreach($posts as $post) {
    // print $post
  }
}