Retrieve newly created Term tid
Why on earth doesn't it return the newly created Entity/Term?!?


Why would it? You already have the term object, why do you need it to be chained in a return from a save method that can and does provide you with extra context??

Just use this:

$term = \Drupal\taxonomy\Entity\Term::create([
  'vid' =&gt; 'subject',
  'langcode' =&gt; 'en',
  'name' =&gt; 'Test af Tag',
  'description' =&gt; [
    'value' =&gt; '&lt;p&gt;My description.&lt;/p&gt;',
    'format' =&gt; 'full_html',
  ],
  'weight' =&gt; -1,
  'parent' =&gt; array(0),
]);

$inserted_or_updated = $term-&gt;save();

$tid = $term-&gt;id();