Well, it wasn't there when I first learned SQL by reading the PostgreSQL manual. I recall that this was back when the size of TEXT columns was limited, so it was before version 7.1 (which introduced TOAST tables), released 13 April, 2001.

So yeah, if you first learned SQL around 25 years ago, something added only 15 years ago is "somewhat recent". And according to a link posted in another comment, other databases introduced this feature (non-recursive WITH clauses) even later.