ERROR: currval of sequence "rss_sub_source_id_seq" is not yet defined in this session
The functions currval(regclass) and lastval() only make sense after nextval() has been used in the same session (not transaction!). Concurrency control in PostgreSQL is reason behind this behavior.
If you are just curious about the current state of the sequence (not the last serial ID actually used in the session), you can always just call nextval():
SELECT nextval(pg_get_serial_sequence('foo', 'id'));

Returns the next serial ID, which had not been used, yet. This carries the flaw of actually consuming the returned ID (which should not matter in a proper relational design). But that side effect can be avoided. In Postgres, serial and IDENTITY columns are implemented using a SEQUENCE internally, which happens to be a table-like object that can be queried like a table. If you are curious about the current state of a SEQUENCE, just use SELECT:
SELECT * FROM public.rss_sub_source_id_seq;

(Requires the SELECT privilege on the sequence, while the above mentioned sequence functions require the USAGE privilege.)
You need to know the name of the sequence for this, and you already know how to get it: pg_get_serial_sequence('rss_sub_source', 'id'). It's easy enough to guess, too. See:

PostgreSQL next value of the sequences?

For ad-hoc use, just compile the query by hand. Or you can use \gexec in psql:
SELECT 'SELECT last_value - CASE WHEN is_called THEN 0 ELSE 1 END FROM ' || pg_get_serial_sequence('b2', 'big_id')\gexec

See:

Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?

For automated (or regular) use, you need dynamic SQL to use the text value returned by the function as SQL identifier. Here is a simple custom function to do that. Also factored in is_called:
CREATE OR REPLACE FUNCTION sequence_currval(_tbl text, _col text, OUT currval bigint)
  LANGUAGE plpgsql AS
$func$
BEGIN
  EXECUTE 'SELECT last_value - CASE WHEN is_called THEN 0 ELSE 1 END FROM ' || pg_get_serial_sequence(_tbl, _col)
  INTO currval;
END
$func$;

Call:
SELECT sequence_currval('foo', 'id');

This is safe against SQLi because pg_get_serial_sequence() returns the identifier safely double-quoted as needed.
The function returns the last value that has been consumed from this sequence. (Does not mean it made its way into the table.) The next one will be greater. (Assumes a default sequence with INCREMENT 1!)
It does not consume a serial ID and does not require that nextval() has been called in the same session.
Consider this demo:
db&lt;&gt;fiddle here