harfbuzz-pure is not setting the HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT or HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_TATWEEL flags, which means that the corresponding GlyphInfo flags will always be False.

Please update the documentation for the unsafeToConcat and safeToInsertTatweel flags to mention that they are unused and will always be False.

I would also suggest leaving a link to the documentation at https://harfbuzz.github.io/harfbuzz-hb-buffer.html#hb-buffer-flags-t in the source file in case you want to enable this feature in the future.