PostgreSQL predicate `IS OF` and `IS NOT OF` supported or documented?
It's not documented, yet, but it is certainly supported and should be moving forward. It's actually in the SQL 2011 spec as type predicate.

8.19 &lt;type predicate&gt;
Function
Specify a type test.

Format
&lt;type predicate&gt; ::=
&lt;row value predicand&gt; &lt;type predicate part 2&gt;

&lt;type predicate part 2&gt; ::=
IS [ NOT ] OF &lt;left paren&gt; &lt;type list&gt; &lt;right paren&gt;

&lt;type list&gt; ::=
&lt;user-defined type specification&gt;
[ { &lt;comma&gt; &lt;user-defined type specification&gt; }... ]