I am looking at Palais Garnier website and is cautions that certain things are not allowed, including "Luggage and travel bags,:" What about small backpacks, think of the size of Rick Steves' daypack. THanks much.
Even a small backpack is considered a travel bag.
I don’t know of any museum in Paris that allows bags inside.
In most museum settings, you will be asked to carry a daypack on your front or off one shoulder, never on the back. I have no personal experience with a daypack at the Garnier. However, since activists defaced artwork in museums, the Minister of Culture has decreed a more thorough search of all bags and no liquids. Recently, we had to remove pens.
The three times that I have been in Palais Garnier, last time Dec 2021, I was able to keep my satchel. Draped over my shoulder and resting against my left hip/leg. The same for the Louvre, Orsay, L'Orangie and all other museums that I have visited.
Unless you want/need for it to be on you, you might check it at the coat check.
And a trick for couples; more than once I have been fine with a small daypack over one shoulder under my arm like a purse when my husband was turned away for the same pack on his shoulder. Women carry purses and sometimes thus can get away with this more, especially if it is over the shoulder and under the arm and thus doesn't shout backpack.
Thank all of you for help and suggestions