After enabling utf8mb4 on 7.50, cannot enable update module
Hacking core, not a great answer.  This is just the best I have found so far.

In modules/update/update.install:

/**
 * Implements hook_schema().
 */
function update_schema() {
  $schema['cache_update'] = drupal_get_schema_unprocessed('system', 'cache');
  $schema['cache_update']['description'] = 'Cache table for the Update module to store information about available releases, fetched from central server.';
  return $schema;
}


So I went to modules/system/system.install:

  $schema['cache'] = array(
    'description' =&gt; 'Generic cache table for caching things not separated out into their own tables. Contributed modules may also use this to store cached items.',
    'fields' =&gt; array(
      'cid' =&gt; array(
        'description' =&gt; 'Primary Key: Unique cache ID.',
        'type' =&gt; 'varchar',
        'length' =&gt; 255,
        'not null' =&gt; TRUE,
        'default' =&gt; '',
      ),
      'data' =&gt; array(
        'description' =&gt; 'A collection of data to cache.',
        'type' =&gt; 'blob',
        'not null' =&gt; FALSE,
        'size' =&gt; 'big',
      ),
      'expire' =&gt; array(
        'description' =&gt; 'A Unix timestamp indicating when the cache entry should expire, or 0 for never.',
        'type' =&gt; 'int',
        'not null' =&gt; TRUE,
        'default' =&gt; 0,
      ),
      'created' =&gt; array(
        'description' =&gt; 'A Unix timestamp indicating when the cache entry was created.',
        'type' =&gt; 'int',
        'not null' =&gt; TRUE,
        'default' =&gt; 0,
      ),
      'serialized' =&gt; array(
        'description' =&gt; 'A flag to indicate whether content is serialized (1) or not (0).',
        'type' =&gt; 'int',
        'size' =&gt; 'small',
        'not null' =&gt; TRUE,
        'default' =&gt; 0,
      ),
    ),
    'indexes' =&gt; array(
      'expire' =&gt; array('expire'),
    ),
    'primary key' =&gt; array('cid'),
  );


And I changed the length of the varchar for the primary key from 255 to 191.  Now I can enable and disable the update module properly.