Another one missed, quite a long time ago, too..:
On 4th of November 2016, Kevin Grittner committed patch:
Implement syntax for transition tables in AFTER triggers. This is infrastructure for the complete SQL standard feature. No support is included at this point for execution nodes or PLs. The intent is to add that soon. As this patch leaves things, standard syntax can create tuplestores to contain old and/or new versions of rows affected by a statement. References to these tuplestores are in the TriggerData structure. C triggers can access the tuplestores directly, so they are usable, but they cannot yet be referenced within a SQL statement.
Continue reading Waiting for PostgreSQL 10 – Implement syntax for transition tables in AFTER triggers.