The core of that complaint is that we'd have to make ASSERT a plpgsql reserved word, which is true enough as things stand today. However, why is it that plpgsql statement-introducing keywords need to be reserved? The only reason for that AFAICS is to allow us to distinguish the statements from assignments. But it seems like that could possibly be gotten around with some work. The first thing that comes to mind is for the lexer to do one-token lookahead and assume that the second token of an assignment must be ":=" (aka "="), ".", or "[". (Have I missed any cases?) Then, any statement for which the second token can't be one of those, which is surely most if not all of them, could have an unreserved leading keyword. This would at a stroke dereserve about half of plpgsql's existing reserved words, as well as remove a roadblock for ASSERT and other new statements.
Doesn't it close a doors to implement a procedures call without explicit CALL statement (like PL/SQL) ?
Personally I doesn't feel to introduce lot of new keywords (statements) to plpgsql. Probably only two - ASSERT (assertions), PRAGMA (some cooperation with plpgsql extensions).