https://www.cyclingnews.com/news/uci-bans-forearms-on-bars-aero-position-adds-penalties-for-littering/After
banning the so-called 'super-tuck' position, the UCI is likely to anger riders in the professional peloton even further with a series of other new rules and punishments, including banning the forearms-on-bars aerodynamic position commonly used in breakaways.
.... The UCI announced the banning of the ‘super-tuck' position last Thursday and updated its rules on Monday, saying: "Riders must observe the standard position as defined by article 1.3.008. Sitting on the bicycle’s top tube is prohibited. Furthermore, using the forearms as a point of support on the handlebar is prohibited except in time trials."
... Article 1.3.008 states: "The rider shall normally assume a sitting position on the bicycle. This position requires that the only points of support are the following:
the feet on the pedals,
the hands on the handlebars
and the seat on the saddle."