Checking of constraints via subqueries?

Hallo all,

in part VI (the reference), the online manual for 8.0.1 states for command
CREATE DOMAIN for parameter
    CHECK (expression)
that currently
     "...CHECK expressions cannot contain subqueries nor refer to
     variables other than VALUE."

So, is this an item for the TODO list?  I can't find it there.  Or do I
overlook something?
This is also stated in the page about CREATE TABLE.

Best regards,
    Wolfgang