Why Was Parallel Processing Removed From Transformers in FME2019?

I read the following on https://knowledge.safe.com/articles/1211/parallel-processing.html.

"As of FME2019, the Parallel Processing options have been removed from most transformers and exist only in the custom transformer infrastructure. A separate article exists to explain setting up parallel processing in a custom transformer."

Why was this change made?

Thanks.