If your parsing code cares about grapheme cluster boundaries, it'll behave differently on the same string when the unicode spec changes. So it may work differen

If your parsing code cares about grapheme cluster boundaries, it'll behave differently on the same string when the unicode spec changes. So it may work differently on different operating systems, or with different versions of your unicode libraries. And that means the output of your validation methods or tokenizers will randomly change! I'm happy for the rendering of strings to change over time. But in my business logic, compilers and database code: That sounds like an unnecessary source of bugs.