Oracle 19c, CDB/PDB, why is DBMS_STANDARD not accessible in a MOUNTED open_mode PDB?

.. if other packages, like DBMS_OUTPUT are accessible (from the hosting CDB) just fine?

I want to write a small piece of PLS code to pre-check, that when I connect as sysdba via OS auth (locally from the DB server) I am connected to the correct PDB, and that the PDB is in one of the expected open_modes, based on a select from v$pdbs, and raise a custom worded exception (and exit the sql script prematurely) when either of the 2 are not present as expected.

So that:

a) my script is not by mistake executed on the CDB, as it is meant to be executed on the PDB