SQL> alter session set standby_max_data_delay=1;

Session altered.

SQL> show parameter standby_max_data_delay
SQL> select * from v$parameter where name='standby_max_data_delay';

no rows selected

SQL>

Unfortunately nowhere because its not a "real" parameter, but a session setting

https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/ALTER-SESSION.html#GUID-DC7B8CDD-4F89-40CC-875F-F70F673711D4

Even tinkering about with some dumps didnt really expose it any meaningful way, and I wasn't about to hex hunting to look for it :-)

We'll log an enhancement to get it exposed in v$session or via sys_context