Issue #3280023 Added "Lowercase plugin" and unit test.

src/Plugin/token_modifier/Lowercase.php

+<?php
+
+namespace Drupal\token_modifier\Plugin\token_modifier;
+
+use Drupal\token_modifier\Plugin\TokenModifierPluginBase;
+
+/**
+ * Makes all characters of the token lowercase.
+ *
+ * @TokenModifier(
+ *   id = "lowercase",
+ *   name = @Translation("Lowercase"),
+ *   description = @Translation("Lowercase the returned string.")
+ * )
+ */
+class Lowercase extends TokenModifierPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform(string $text, array $data = [], array $options = []) {
+    return strtolower($this->token->replace($text, $data, $options));
+  }
+
+}

tests/src/Unit/LowercaseTest.php

+<?php
+
+namespace Drupal\Tests\token_modifier\Unit;
+
+use Drupal\token_modifier\Plugin\token_modifier\Lowercase;
+
+class LowercaseTest extends TokenModifierTestCase {
+
+  /**
+   * Tests the Lowercase token modifier.
+   *
+   * @param string $value
+   *   The source value for the modifier.
+   * @param string $expected
+   *   The expected result.
+   *
+   * @covers ::transform
+   *
+   * @dataProvider providerTestTransform
+   */
+  public function testTransform(string $value, string $expected) {
+    $plugin = new Lowercase([], 'lowercase', [], $this->token);
+    $actual = $plugin->transform($value);
+    $this->assertSame($expected, $actual);
+  }
+
+  /**
+   * Provides data for testTransform.
+   */
+  public function providerTestTransform(): array {
+    $data['basic'] = ['Hello World!', 'hello world!'];
+    $data['already lowercased'] = ['working?', 'working?'];
+    $data['capitalised'] = ['CAPITALISED', 'capitalised'];
+    return $data;
+  }
+
+}