The question is if `font-lock-syntactic-keywords' really should be
declared obsolete.
From docu of `syntax-propertize-function':
"The specified function may call `syntax-ppss' on any position
before END, but it should not call `syntax-ppss-flush-cache',
which means that it should not call `syntax-ppss' on some
position and later modify the buffer on some earlier position."
So "on any position" but not "on some position"?
IMHO that's not ready.
That presumes that font-lock-syntactic-keywords does not suffer from
tricky interactions with syntax-ppss. Of course, that's not the case,
instead these are simply undocumented.
Stefan