7 files declare their use of FormHelper in Drupal 8

common.inc in core/includes/common.inc

Common functions that many Drupal modules will need to reference.

EntityField.php in core/modules/views/src/Plugin/views/field/EntityField.php

FieldBlock.php in core/modules/layout_builder/src/Plugin/Block/FieldBlock.php

FilterPluginBase.php in core/modules/views/src/Plugin/views/filter/FilterPluginBase.php

FormHelperTest.php in core/tests/Drupal/Tests/Core/Form/FormHelperTest.php

Renderer.php in core/lib/Drupal/Core/Render/Renderer.php

RendererLegacyTest.php in core/tests/Drupal/KernelTests/Core/Render/RendererLegacyTest.php