I messed around with your changes to the grammar and it seems you don't need to add PERCENT as a reserved keyword. Moving this to the unreserved keyword section does not cause any shift/reduce errors, and the regression tests still pass. Relative to your patch v4, these changes help:
In sql standard PERCENT list as reserved world so i don't think it is a thing that can change by me.
I think the main reason create_table fail in test_ddl_deparse is because i don't surround PERCENT key word in
