I don't see much use for this because the documentation says that "server's execute command cannot be used directly within pl/pgsql function (and it is not needed). Within pl/pgsql you can execute update/delete commands using pl/pgsql EXECUTE command and get results like row_count using "get diagnostic".
Why would somebody do what you have shown in your example in pl/pgsql? Or do you have a more general use-case for this enhancement?
I found that such a statement would get 0 in PL/pgSQL.
PREPARE smt_del(int) AS DELETE FROM t1; EXECUTE 'EXECUTE smt_del(100)'; GET DIAGNOSTICS j = ROW_COUNT;
In fact, this is a problem with SPI, it does not support getting result of the EXECUTE command. I made a little enhancement. Support for the number of rows processed when executing INSERT/UPDATE/DELETE statements dynamically.