I tried to book the Pantheon for our trip in October and our dates were blocked out. I messaged them directly and received this response below:
Thank you for your interest in national monuments.
In response to your request, we would like to inform you that, due to the enshrinement of Robert Badinter, the Panthéon will be closed from September 29 to October 10, 2025, and will then be free to all from October 11 to 13, 2025.
Therefore, the online sales site is not accepting reservations for this event.
FYI. Hopefully we will be able to visit it on another trip to Paris.