How do I get the current taxonomy term from the plug-in context?
Update 10.05.2019:

There is an unresolved issue about that on drupal.org: 
Term route context.

It uses the same solution as in my answer below.

Original answer:

You can create a custom context provider "Taxonomy term from URL".

service definition in MYMODULE.services.yml:

  MYMODULE.taxonomy_term_context:
    class: Drupal\MYMODULE\ContextProvider\TaxonomyTermContext
    arguments: ['@current_route_match']
    tags:
      - { name: 'context_provider' }


The /src/ContextProvider/TaxonomyTermContext.php

&lt;?php

namespace Drupal\MYMODULE\ContextProvider;

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Plugin\Context\Context;
use Drupal\Core\Plugin\Context\ContextDefinition;
use Drupal\Core\Plugin\Context\ContextProviderInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\StringTranslation\StringTranslationTrait;

/**
 * Sets the current taxonomy term as a context on taxonomy term routes.
 */
class TaxonomyTermContext implements ContextProviderInterface {

  use StringTranslationTrait;

  /**
   * The route match object.
   *
   * @var \Drupal\Core\Routing\RouteMatchInterface
   */
  protected $routeMatch;

  /**
   * Constructs a new object.
   *
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The route match object.
   */
  public function __construct(RouteMatchInterface $route_match) {
    $this-&gt;routeMatch = $route_match;
  }

  /**
   * {@inheritdoc}
   */
  public function getRuntimeContexts(array $unqualified_context_ids) {
    $result = [];
    $context_definition = new ContextDefinition('entity:taxonomy_term', $this-&gt;t('Taxonomy term from URL'));
    $value = NULL;
    if (
      ($route_object = $this-&gt;routeMatch-&gt;getRouteObject())
      &amp;&amp; ($route_contexts = $route_object-&gt;getOption('parameters'))
      &amp;&amp; isset($route_contexts['taxonomy_term'])
    ) {
      if ($term = $this-&gt;routeMatch-&gt;getParameter('taxonomy_term')) {
        $value = $term;
      }
    }

    $cacheability = new CacheableMetadata();
    $cacheability-&gt;setCacheContexts(['route']);

    $context = new Context($context_definition, $value);
    $context-&gt;addCacheableDependency($cacheability);
    $result['taxonomy_term'] = $context;

    return $result;
  }

  /**
   * {@inheritdoc}
   */
  public function getAvailableContexts() {
    $context = new Context(new ContextDefinition('entity:taxonomy_term', $this-&gt;t('Taxonomy term from URL')));
    return ['taxonomy_term' =&gt; $context];
  }

}


The ctools module provides already the condition plugin for entity bundles:ctools/src/Plugin/Condition/EntityBundle.php. You can use it together with the new service.

See also config block visibility by vocabulary, but it not show block access fail(miss context).