It is not used like that? I found this one: https://github.com/rchl/HTMLPlusGritSyntax/blob/a939f4a42593c0a0e9b0a4f168b67f0ae15c3379/HTML-grit.sublime-syntax#L27
Error loading syntax file
“Packages/SublimeAmxxPawn/AmxxPawn.sublime-syntax”:
expected match or include at line 250 column 9
You also probably don’t want true there. It needs to be a numeric value, indicating the number of scopes to pop off the stack.
The syntax documentation says:
clear_scopes. This setting allows removing scope names from the current stack. It can be an integer, or the value true to remove all scope names.
So a value of true is valid if your intention is to remove all of the scopes. Whether or not that’s what you intended to do is another matter, though. 