Flask THREADS_PER_PAGE configuration
The parameter is not used by the current Flask version, nor was it ever used in the past.
Flask is a WSGI app, and relies on the WSGI server to handle concurrency. For development the Werkzeug server is used, but it too does not use that configuration variable.
The only reference I can find to the parameter was on a now deleted Flask project Wiki page; the parameter is mentioned without further comment. All other sources appear to have taken it from there.