Dress Code

Most cathedrals in Europe do enforce dress codes of no exposed knees and shoulders.

There is no dress code at Versailles. I wouldn’t wear flip flops for my footwear but that is because they would be uncomfortable for me. It’s not a church or former church and it’s so packed no one will notice your attire. There is no dress code at Sacre Coeur but visitors are asked to wear “appropriate” clothing, which is typically covered shoulders and hems below the knees even for shorts. Open toe shoes are not a concern anywhere you mention.

Only place I have encountered dress codes that are enforced for visiting churches is in Italy.

It's strange... I assume whatever god they worship has actually seen human flesh before...

The cathedral in Barcelona (not La Sagrada Familia) enforces its dress code. Orthodox churches (which are mostly in the east) often have prominent signage about appropriate attire. I don't know to what degree they enforce their posted rules, but you'll definitely see that worshippers are covered up, including women wearing scarves.

Churches in France are owned by the state and leased to the Catholic church, so they're nowhere near as strict as Italy...

We were at St. Mark’s in June on a very hot & humid day. I made my family follow the “cover knees & shoulders” rule. Plenty of people ignored the rule and it wasn’t being enforced. Maybe due to the heat? If I had to do it over again I would have thrown a couple light scarfs in the bag and everyone would wear what’s comfortable. If they enforce the rule, wrap a scarf around your waist.

That said, when we were there in June 2018, the rule was being enforced and paper cover ups were being sold at the entrance.

No rules about open toed sandals.

Simon, dress code is a sign of respect. You may not be aware of the recent controversy about proper dress for US Congressmen.

Simon, I am sure you did not intend any snark about dress codes.
In visiting churches, temples or mosques, I always feel it is good manners to honor and respect the suggestions and/or rules of each place, just as I would expect guests in my home to be courteous and respectful there. I follow the rules about dress, noise, cameras in museums, churches, castles, libraries, etc. and appreciate the opportunity to see what there is to see in these places. It is poor manners to flaunt such rules, especially if it is only to make a statement.

What's with all these lessons towards Simon? He was correct: the only dress code of the sites mentioned is in Italy.

Without reading all the answers, were the questions about Saint Mark's answered? Sandals -ok, shorts cover knees-yes, women's shoulders--don't remember but probably.

Whenever I see discussions like this I wonder how those who appear flummoxed about respectful attire in European houses of worship, would feel about people who came to their own house of worship in shorts, tank tops, etc. Like Stan said, "It may be a tourist attraction to many, but it's meaningful to others whether they chastise you for it or not."

Personally, I don’t believe in any supernatural entities, belong to any kind of religion or attend any kind of house of worship. But I understand that others do, respect their religious choices and feel very lucky to be able to see the magnificent places where they worship. Dressing respectfully is a very small price to pay for that privilege.

FYI, the OP traveled back in August and has long been back. 😊 Here is her TR if anyone is interested. https://community.ricksteves.com/travel-forum/france/our-trip-with-11-adults-and-2-young-children