PostCreate/Hook on User Registration
It depends from what exactly the code needs to achieve.
hook_form_alter() and hook_form_FORM_ID_alter() are still used from Drupal 9, and they can be used to hook into account registration before users can enter values for the account to create. They should not be used to add an entity field, or add a validation handler for an entity, for which hook_entity_base_field_info() and hook_entity_bundle_field_info() (to add entity fields to an entity) or hook_entity_type_alter() (to add constrains to validate an entity) are preferable.
If you are instead looking for the Drupal 9 equivalent of Drupal 7 hook_user_insert(), hook_user_update(), and hook_user_presave(), which are invoked after the users entered the account data (and they should not be used to invalidate the entered data), they are still used, but they are documented under hook_ENTITY_TYPE_insert(), hook_ENTITY_TYPE_update(), and hook_ENTITY_TYPE_presave(). When you implement those hooks, you replace ENTITY_TYPE with user, for the User entity. This means that the name of the function implementing those hooks for the User entity doesn't change respect of the function name used in Drupal 7; it's just a matter of documentation.
Note also that the generic hooks, invoked for every entity implemented in Drupal, for example hook_entity_insert() are still used in Drupal 9. (See hook_entity_insert().) The documentation page for that hook for Drupal 9 contains a reference to the equivalent hook invoked for a single entity type (See also hook_ENTITY_TYPE_insert(), written on two lines).
Further reading

Drupal Entity Validation API: Create Custom Entity Constraints
Documentation for entity hooks has been consolidated