pg_class has no toast table?

Still fooling with VACUUM FULL on catalogs ... I find that a sanity
check I put in is barfing on "VACUUM FULL pg_class", because the
transient table is built with a toast table, whereas pg_class hasn't got
one.  It seems like it probably ought to have one, because either relacl
or reloptions could in principle be too big to fit without toasting
(which is exactly why AlterTableCreateToastTable thinks it should make
one for the transient table).

I have a vague feeling that we intentionally omitted a toast table for
pg_class, but I don't remember why.  Comments?
        regards, tom lane