How could we verify that the system ID is a valid one, except based on its format? It’s not possible, unless using one of the character as a character control. Maybe it’s what you meant.
For HTMLEXE 5, we’re still working on it (remember it’s a very huge update, requiring us to rewrite a lot of parts and make a lot of testing) and it will be renamed HTML Executable 2018. With the new Windows 10 release scheme, it’s better to switch to release names based on years.