Unrecognized exception condition "deprecated_feature"

For some reason using an error code of 'deprecated_feature' does not work,
while its equivalent '01P01' works just fine.

kouber=# DO $$
BEGIN
RAISE WARNING 'Deprecated'
USING ERRCODE = 'deprecated_feature';
END;
$$;
ERROR: unrecognized exception condition "deprecated_feature"
CONTEXT: PL/pgSQL function inline_code_block line 3 at RAISE

kouber=# DO $$
BEGIN
RAISE WARNING 'Deprecated'
USING ERRCODE = '01P01';
END;
$$;
WARNING: Deprecated
DO

The thing that is making this issue quite strange is that I see that string
well defined in the source code (and copy/pasted it from there, as well as
from the docs, yet the result remains the same).

kouber(at)spectre:~/src/postgres$ grep -r deprecated_feature .
./src/backend/utils/errcodes.txt:01P01 W
ERRCODE_WARNING_DEPRECATED_FEATURE
deprecated_feature

I am using PostgreSQL 10.1, I apologize in case it has already been fixed
in 10.2 or 10.3.

Regards,
--
Kouber Saparev