Currently there's no support in Teiid for default values for parameters of a Function.

But Function element in BNF for SQL Grammar shares the definition with Procedure, which permits to deploy such a VDB, where there are DEFAULT values defined on a Function parameter.

So a VDB with following DDL shouldn't deploy successfully:

CREATE VIRTUAL FUNCTION f1(e2 string NOT NULL DEFAULT 'default') RETURNS string AS
BEGIN
    RETURN e2;
END;