I can not find any document regarding to shared_heap_size
Based on the description, this is related to Amazon Aurora product, and not PostgreSQL by itself.

See https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora.AuroraPostgreSQL.html


  Amazon Aurora PostgreSQL is a fully managed, PostgreSQL-compatible, relational database engine that combines the speed and reliability of high-end commercial databases with the simplicity and cost-effectiveness of open-source databases. Aurora PostgreSQL is a drop-in replacement for PostgreSQL


So Amazon added some settings.

You will need to contact them directly and ask since their documentation does not provide a result when doing a search, https://docs.aws.amazon.com/search/doc-search.html?searchPath=documentation-guide&amp;searchQuery=shared_heap_size&amp;x=0&amp;y=0&amp;this_doc_product=Amazon+Relational+Database+Service&amp;this_doc_guide=User+Guide&amp;doc_locale=en_us#facet_doc_product=Amazon%20Relational%20Database%20Service&amp;facet_doc_guide=User%20Guide gives Your search for "shared_heap_size" returned no results.