BTW, we should also try to conclude on my yesterday's point as to why it is okay to have the same behavior for default_tablespace and default_table_access_method and not for this parameter? I am asking because if we change the current behavior, tomorrow, we can get complaints that one expects the old behaviour as that was similar to other GUCs like default_tablespace and default_table_access_method.
Fair point. I haven't examined the validation of GUCs in parallel workers closely, but one argument for preventing parallel workers from failing due to an incorrect value of synchronized_standby_slots is that a select query works in this situation without parallel workers.
Whereas, for incorrect values of default_tablespace and default_table_access_method, most commands would fail regardless of whether parallel workers are enabled.
PFA a test for the original bug report on this thread. This applies on the v3 version of the patch that was shared.