With a view of flaggings, how do I get the flagged entity?
Whoops, this is a work in progress to be added to Flag at some point.
In the meantime, this can be worked around using an entity reference field.
First, add an entity reference field to the flag.
Next, set the entity reference using hook_flagging_presave().
function MYMODULE_flagging_presave(FlaggingInterface $flagging) {
  switch ($flagging-&gt;getFlagId()) {
    case 'MYFLAGTYPE':
      if ($flagging-&gt;isNew()) {
        // Set entity reference for new flaggings.
        $flagged_entity_id = $flagging-&gt;get('entity_id')-&gt;value;
        $flagging-&gt;set('field_myflag_entityreference', $flagged_entity_id);
      }
    break;
  }
}

Finally, you can use the entity reference to the flagged node in your view in the same way as any other entity reference.
Because the flag module patch to solve this issue is in flux, it seems to me a bit dangerous to apply the patch to a production site, because the ultimate direction of the flag module may differ from any given patch. So, using an entity reference has the advantage of achieving the desired solution now (get access to the flagged entity in the flaggings view) while still allowing the future flag solution to be used (when flag becomes able to support this, you can simply delete the entity reference field, as it will no longer be necessary).