The \Drupal\search_api\Utility\PostRequestIndexing::removeFromIndexing() method was added in #3325206: Bug when an item is first updated then ignored during a single page request to solve a problem when a specific item was tracked as both inserted and then deleted in a single page request. However, while the method on the interface has a type hint string $index_id for the first parameter, this was accidentally omitted in the child method (which happens to be permitted by PHP).
The type hint will, however, be added in version 2.0.0, calling the method with a non-string $index_id is therefore deprecated.
Since calling the method with $index_id = NULL does not make sense in any case, simply remove such calls from your code (possibly by explicitly checking for NULL values before calling).
