It has been tried in the past by riders such as Tadej Pogacar and Jonas Vingegaard and although it's not banned, it's being criticized by more and more people. Carbon monoxide rebreathing is still a relatively unknown practice in the world of cycling but there are vague arguments that it could help with performance enhancing. The manager of a French team was quite critical of this idea.
Jean-René Bernaudeau, manager of TotalEnergies, did not spare on the words in an interview with VeloFute: "The attractiveness and credibility of the sport are at stake. Look at Pogacar; I like him as a rider; he's nice, but this news about carbon monoxide rebreathers? That damages cycling's reputation."
This year we have seen substantial increase in the climbing performances of a few riders including a Pogacar that has won the Tour de France, Giro d'Italia, World Championships and two monuments amongst other wins. Bernaudeau argues that the credibility of these performances is not the highest but also that having such a dominant rider at the top is a hit for the sport's attractiveness - as is argued occasionally that Mathieu van der Poel has the same effect in cyclocross.
In the same interview Burgaudeau talked about the UCI points, criticizing them, and also how the team was not able to sign Julian Alaphilipe despite having quite a good offer on the table. Ultimately, the French manager still has a traditional point of view on the sport but at the moment that continues to fit very well with it's calendar and ambitions.
"Some teams buy their place in the WorldTour. Riders' contracts are also increasingly being bought off. That doesn't suit us," he states. "We want to educate, recruit, and develop a project. As we do with the sports studies in La Roche-sur-Yon, Vendée U, and our pro team, my sponsor could ask me to win the Tour by buying riders with a lot of power and points, but that has no taste. That's not who we are and not what my sponsor wants."