ALTER TABLE SET (toast.asdf) is not rejected

I noticed that this is accepted:

postgres=# ALTER TABLE t SET (toast.asdf=128);
ALTER TABLE

I thought since "toast" was a core namespace, it would've been rejected?

I recall having read a discussion about verifying these ... I wasn't able
to find what I was thinking of, but found this one.
https://www.postgresql.org/message-id/flat/20090114144332.GF24156%40alvh.no-ip.org

Justin