Given its current definition that depends on time(NULL), timetz_zone

is certainly no longer immutable, but must indeed be marked volatile.
I wonder if it should use the value of now() (that is, transaction
start time) so that it could be marked stable.  But it's probably not
important enough to be worth changing the code for ... indeed, I'm not
even going to force an initdb for this catalog change, seeing that we
just did one a few hours ago.